feat: PDF Viewer translation via context menu and two-mode Action Popupv0.1.0

Add Firefox built-in PDF Viewer support using only public WebExtensions
APIs: a selection context menu ("選択したテキストを翻訳") starts a
translation and the toolbar Action Popup shows the streaming result.
No content-script injection into the privileged PDF Viewer, no PDF.js
internals, no separate windows, no custom viewer (Specifications.md §4.6).

Specification alignment (Phase 1)
- Specifications.md: new requirements PDF-01..04 (PDF selection
  translation), CTX-01..03 (context menu translation), POP-01..06
  (popup two modes); SEL-06 updated to route PDF through the menus API;
  section 13.4 adds a 14-item manual PDF verification checklist; section
  15 removes "Firefox内蔵PDF Viewerへの対応" from out-of-scope
- Design.md: architecture diagram with the menus path, menu-translation
  component, menus permission, popup translation-result state machine
  (idle/starting/streaming/completed/error/cancelled), shared translation
  state (§9.2), context-menu integration phase, test plan and API audit
  checklist items (menus permission scope, no PDF Viewer internals)
- Security.md: menus permission restricted to selection-context
  translation; PDF Viewer treated as privileged browser UI (no injection,
  no wrappedJSObject/cloneInto/exportFunction, no internal DOM access);
  menus.OnClickData.selectionText treated as untrusted input passing the
  same validation pipeline; popup result rendering stays textContent-only

Background (Phase 2)
- translation-service.js (new): the single translation pipeline shared by
  every entry point - selection validation (type/trim/empty/control
  chars/max length), prompt building, Ollama request, session/abort, and
  the shared translation state (requestId, sourceText, translatedText,
  status, model, error, startedAt, source). "source" is metadata only; a
  PDF-specific translation engine does not exist
- menu-translation.js (new): validates menus.OnClickData.selectionText
  and starts a sink-less translation observed via GET_POPUP_STATE
- message-router.js: routes port translations through the shared service;
  new runtime messages GET_POPUP_STATE and REGENERATE_LAST; popup-state
  port handler broadcasts live state (initial push + every chunk/status
  change) so an open popup shows streaming progress in real time
- index.js: registers the selection context menu (contexts: ["selection"]
  only, no menus.onShown) and calls browser.action.openPopup()
  synchronously inside the click user gesture, catching failures so a
  popup-open problem never breaks the translation itself
- manifest.json: permissions now ["storage", "menus"]
- settings-service.js: __resetCacheForTests hook for test isolation

Popup two-mode UI (Phase 3)
- popup.html/js/css: mode nav (設定 / 翻訳). Translation mode shows
  status, model, streaming text (textContent only), user-safe error, and
  copy (Clipboard API only, no execCommand fallback), regenerate (new
  requestId reusing the stored source text) and a settings shortcut.
  Closing the popup never cancels the request; reopening shows the
  latest text and continues receiving chunks live. Initial mode: active
  (starting/streaming) translation wins, otherwise the last tab the user
  picked is restored (storage.local.popupMode, default settings)

Fresh-install model auto-selection
- translation-service.js ensureModelConfigured(): when storage.model is
  empty, pick the first model from the running Ollama and persist it
  before translating, mirroring the popup/options auto-select. A user who
  never opened any settings UI can translate immediately; when Ollama is
  unreachable a clear OLLAMA_UNAVAILABLE message is shown

User-safe error details
- detailForUser(): strips URLs (no internal host names echoed to the UI),
  caps length at 120 chars, drops full JSON response bodies, and hides
  meaningless low-level network errors so the popover/popup shows the
  base message plus at most a short, safe detail

Tests: 85 passed, 0 failed
- selectionText validation (untrusted input pipeline)
- menu selection -> translation -> popup state streaming/completed
- empty selection rejected without starting a request
- streaming status transitions observed through GET_POPUP_STATE
- Ollama unreachable surfaces a user-safe error into popup state
- newer request replaces older state (abort semantics)
- regenerateLast reuses source text with a fresh requestId
- popup-state port receives live broadcasts; wrong port names ignored
- fresh-install auto-select persists the first Ollama model
- error detail sanitization (URL stripping, length cap, JSON drop)
- popup/options smoke tests updated for the two-mode markup
- setupRouter resets the shared settings cache for test isolation

Docs: README documents the two-mode popup and PDF flow; Testing-Results
records automated results and the manual PDF checklist (to be executed).

No git-side surprises: this commit was created explicitly on request
(requirement 18 of the PDF task had deferred commits until now).

9 files changed, 744 insertions, 126 deletions

diff --git a/src/background/index.js b/src/background/index.js
index 9c97c9d..8e3c5f1 100644
--- a/src/background/index.js
+++ b/src/background/index.js
@@ -5,8 +5,9 @@
  * event page can be woken by any of them (MDN: background scripts).
  */
 
-import { registerRuntimeMessageHandler, handleConnection } from './message-router.js';
+import { registerRuntimeMessageHandler, handleConnection, handlePopupStateConnection } from './message-router.js';
 import { loadSettings, watchSettings } from './settings-service.js';
+import { startFromSelection } from './menu-translation.js';
 
 // One-shot messages from popup / options.
 registerRuntimeMessageHandler();
@@ -14,6 +15,35 @@ registerRuntimeMessageHandler();
 // Streaming connections from the content script.
 browser.runtime.onConnect.addListener(handleConnection);
 
+// Live translation state updates for the popup (translation result mode).
+browser.runtime.onConnect.addListener(handlePopupStateConnection);
+
+// Selection context menu (works on normal pages and the PDF Viewer).
+if (browser.menus && typeof browser.menus.create === 'function') {
+  browser.menus.create({
+    id: 'translate-selection',
+    title: '選択したテキストを翻訳',
+    contexts: ['selection']
+  });
+  browser.menus.onClicked.addListener((info) => {
+    if (!info || info.menuItemId !== 'translate-selection') return;
+    const result = startFromSelection(info.selectionText);
+    if (result.ok) {
+      // Open the toolbar popup as the result UI. openPopup() must be called
+      // synchronously within the click user gesture. It can reject when the
+      // popup is already open or on unsupported platforms - ignore safely.
+      try {
+        const p = browser.action.openPopup();
+        if (p && typeof p.catch === 'function') {
+          p.catch(() => {});
+        }
+      } catch {
+        // openPopup unavailable; the user can open the popup manually
+      }
+    }
+  });
+}
+
 // Keep the validated settings cache in sync and load initial settings.
 watchSettings();
 void loadSettings();
\ No newline at end of file
diff --git a/src/background/menu-translation.js b/src/background/menu-translation.js
new file mode 100644
index 0000000..1465c53
--- /dev/null
+++ b/src/background/menu-translation.js
@@ -0,0 +1,28 @@
+/**
+ * Context-menu translation entry (background).
+ *
+ * Receives the raw `menus.OnClickData.selectionText`, validates it through
+ * the shared pipeline and starts a translation without a sink. The popup
+ * observes results via the shared translation state (GET_POPUP_STATE).
+ */
+
+import { validateSelectionText, runTranslation, messageForCode } from './translation-service.js';
+import { createRequestId } from '../shared/messages.js';
+import { ERROR_CODES } from '../shared/constants.js';
+
+/**
+ * Start a translation from a context-menu selection.
+ * @param {*} selectionText raw value from menus.OnClickData
+ * @returns {{ok: boolean, requestId?: string, error?: string, message?: string}}
+ */
+export function startFromSelection(selectionText) {
+  const validated = validateSelectionText(selectionText);
+  if (!validated.ok) {
+    return { ok: false, error: validated.code, message: messageForCode(validated.code) };
+  }
+  const requestId = createRequestId();
+  // No sink: the popup polls GET_POPUP_STATE and receives streaming updates
+  // through the shared translation state.
+  void runTranslation({ text: validated.text, requestId, source: 'context-menu', sink: null });
+  return { ok: true, requestId };
+}
\ No newline at end of file
diff --git a/src/background/message-router.js b/src/background/message-router.js
index 9428312..cb9b69c 100644
--- a/src/background/message-router.js
+++ b/src/background/message-router.js
@@ -3,43 +3,25 @@
  *
  * - runtime.Port ('translation-stream'): translate / cancel requests from
  *   the content script, streaming started/chunk/completed/error back.
- * - runtime.onMessage: one-shot listModels request for popup/options.
+ * - runtime.onMessage: one-shot requests for popup/options:
+ *     - listModels
+ *     - getPopupState (popup translation result mode)
+ *     - regenerateLast
  *
- * All inbound messages are validated; invalid ones are discarded
- * (Specifications.md SEC-06, Design.md §8.4).
+ * The actual translation pipeline (validation, prompt, Ollama request,
+ * session/abort, shared state) lives in translation-service.js and is used
+ * by BOTH the content-script port flow and the context-menu flow, so there
+ * is a single request-generation implementation (Design.md §9.2).
  */
 
 import {
-  CONTENT_TO_BG,
-  BG_TO_CONTENT,
   RUNTIME_MESSAGES,
-  MAX_SELECTION_LENGTH,
   ERROR_CODES
 } from '../shared/constants.js';
-import { isValidTranslateRequest, isValidCancelRequest, isTrustedSender } from '../shared/messages.js';
+import { isValidTranslateRequest, isValidCancelRequest, isTrustedSender, createRequestId } from '../shared/messages.js';
 import { getSettings } from './settings-service.js';
-import { TranslationSession } from './translation-session.js';
-import { chatStream, listModels } from './ollama-client.js';
-import { buildSystemPrompt } from '../shared/prompt.js';
-import { buildOptions, toApiThinking } from '../shared/settings-schema.js';
-
-const session = new TranslationSession();
-
-/** User-facing messages per error code (kept minimal; no stack traces). */
-const ERROR_MESSAGES = {
-  [ERROR_CODES.INVALID_REQUEST]: '要求が不正です。',
-  [ERROR_CODES.TEXT_EMPTY]: '選択文字列が空です。',
-  [ERROR_CODES.TEXT_TOO_LONG]: `選択文字列が長すぎます（上限 ${MAX_SELECTION_LENGTH} 文字）。`,
-  [ERROR_CODES.OLLAMA_UNAVAILABLE]: 'Ollamaに接続できません。Ollamaが起動しているか確認してください。',
-  [ERROR_CODES.MODEL_NOT_FOUND]: '指定されたモデルが見つかりません。設定画面でモデルを選び直してください。',
-  [ERROR_CODES.INVALID_SETTINGS]: '設定値が不正です。設定画面を確認してください。',
-  [ERROR_CODES.REQUEST_ABORTED]: '翻訳を中断しました。',
-  [ERROR_CODES.STREAM_ERROR]: '翻訳中にエラーが発生しました。',
-  [ERROR_CODES.UNKNOWN_ERROR]: '不明なエラーが発生しました。'
-};
-
-/** Message shown when no model is configured at all. */
-const NO_MODEL_MESSAGE = 'モデルが設定されていません。ツールバーアイコンまたは設定画面でモデルを選択してください。';
+import { listModels } from './ollama-client.js';
+import { runTranslation, getSession, getTranslationState, addPopupPort, removePopupPort } from './translation-service.js';
 
 /**
  * Handle one content-script port connection.
@@ -52,12 +34,12 @@ export function handleConnection(port) {
     if (!isTrustedSender(port.sender)) return;
 
     if (isValidTranslateRequest(message)) {
-      void runTranslation(port, message.requestId, message.text);
+      void runTranslation({ text: message.text, requestId: message.requestId, source: 'content', sink: port });
       return;
     }
     if (isValidCancelRequest(message)) {
-      if (session.isActive(message.requestId)) {
-        session.clear(message.requestId);
+      if (getSession().isActive(message.requestId)) {
+        getSession().clear(message.requestId);
       }
       return;
     }
@@ -65,84 +47,27 @@ export function handleConnection(port) {
   });
 
   port.onDisconnect.addListener(() => {
-    // If the tab goes away, abort the translation owned by this port.
-    if (session.hasActive() && session.active.port === port) {
-      const requestId = session.active.requestId;
-      session.clear(requestId);
+    // If the tab goes away, clear the session entry owned by this port.
+    // The popup-facing translation state keeps whatever was accumulated so
+    // reopening the popup still shows the latest text.
+    if (getSession().hasActive() && getSession().active.port === port) {
+      const requestId = getSession().active.requestId;
+      getSession().clear(requestId);
     }
   });
 }
 
 /**
- * Run one translation: settings -> prompt -> Ollama stream -> port messages.
- * @param {object} port
- * @param {string} requestId
- * @param {string} text
- */
-async function runTranslation(port, requestId, text) {
-  const settings = await getSettings();
-
-  if (!settings.enabled) {
-    postError(port, requestId, ERROR_CODES.INVALID_REQUEST, 'Translation is disabled');
-    return;
-  }
-  if (!settings.model) {
-    postError(port, requestId, ERROR_CODES.INVALID_SETTINGS, NO_MODEL_MESSAGE);
-    return;
-  }
-
-  const controller = session.start(requestId, port);
-
-  port.postMessage({ type: BG_TO_CONTENT.STARTED, requestId, model: settings.model });
-
-  try {
-    await chatStream({
-      host: settings.ollamaHost,
-      model: settings.model,
-      messages: [
-        { role: 'system', content: buildSystemPrompt(settings) },
-        { role: 'user', content: text }
-      ],
-      think: toApiThinking(settings),
-      options: buildOptions(settings),
-      signal: controller.signal,
-      onChunk: (content) => {
-        // Drop chunks if a newer request took over.
-        if (!session.isActive(requestId)) return;
-        port.postMessage({ type: BG_TO_CONTENT.CHUNK, requestId, text: content });
-      }
-    });
-    if (session.isActive(requestId)) {
-      port.postMessage({ type: BG_TO_CONTENT.COMPLETED, requestId });
-    }
-  } catch (error) {
-    const code = (error && error.code) || ERROR_CODES.UNKNOWN_ERROR;
-    // Aborted requests are reported by TranslationSession.abortActive();
-    // avoid a duplicate error for the same requestId.
-    if (code !== ERROR_CODES.REQUEST_ABORTED && session.isActive(requestId)) {
-      postError(port, requestId, code, error && error.message ? error.message : '');
-    }
-  } finally {
-    session.clear(requestId);
-  }
-}
-
-/**
- * @param {object} port
- * @param {string} requestId
- * @param {string} code
- * @param {string} detail optional concrete reason shown to the user
+ * Handle one popup-state port connection. The popup subscribes to live
+ * translation state updates (Design.md §7.4, §9.2).
+ * @param {object} port runtime.Port
  */
-function postError(port, requestId, code, detail) {
-  const base = ERROR_MESSAGES[code] || ERROR_MESSAGES[ERROR_CODES.UNKNOWN_ERROR];
-  // Append the concrete server/detail reason when provided so the popover
-  // shows WHY it failed (e.g. the Ollama error body for HTTP 400).
-  const message = detail && detail !== base ? `${base}\n(${detail})` : base;
-  try {
-    port.postMessage({ type: BG_TO_CONTENT.ERROR, requestId, code, message });
-  } catch {
-    // port may be disconnected
-  }
+export function handlePopupStateConnection(port) {
+  if (!port || port.name !== 'popup-state') return;
+  addPopupPort(port);
+  port.onDisconnect.addListener(() => {
+    removePopupPort(port);
+  });
 }
 
 /**
@@ -164,6 +89,24 @@ export function registerRuntimeMessageHandler(browserApi) {
         }
       })();
     }
+    if (message && message.type === RUNTIME_MESSAGES.GET_POPUP_STATE) {
+      return Promise.resolve({ ok: true, state: getTranslationState() });
+    }
+    if (message && message.type === RUNTIME_MESSAGES.REGENERATE_LAST) {
+      return (async () => {
+        try {
+          const state = getTranslationState();
+          if (!state.sourceText) {
+            return { ok: false, error: ERROR_CODES.TEXT_EMPTY };
+          }
+          const requestId = createRequestId();
+          void runTranslation({ text: state.sourceText, requestId, source: state.source || 'context-menu', sink: null });
+          return { ok: true, requestId };
+        } catch (error) {
+          return { ok: false, error: (error && error.code) || ERROR_CODES.UNKNOWN_ERROR };
+        }
+      })();
+    }
     return Promise.resolve({ ok: false, error: ERROR_CODES.INVALID_REQUEST });
   });
 }
diff --git a/src/background/settings-service.js b/src/background/settings-service.js
index 2687921..55d2a8f 100644
--- a/src/background/settings-service.js
+++ b/src/background/settings-service.js
@@ -79,4 +79,13 @@ export function watchSettings(browserApi) {
       cache = sanitizeSettings(changes.settings.newValue);
     }
   });
+}
+
+/**
+ * Test-only: clear the module-level settings cache so each test starts from
+ * storage state instead of inheriting the previous test's cache.
+ * @visibleForTesting
+ */
+export function __resetCacheForTests() {
+  cache = null;
 }
\ No newline at end of file
diff --git a/src/background/translation-service.js b/src/background/translation-service.js
new file mode 100644
index 0000000..f443237
--- /dev/null
+++ b/src/background/translation-service.js
@@ -0,0 +1,319 @@
+/**
+ * Translation service (background): shared translation pipeline.
+ *
+ * Both entry points use this module so there is exactly one request
+ * generation / streaming / state implementation:
+ *   - content script (runtime.Port streaming, Design.md §8)
+ *   - context menu / popup (translation state, Design.md §9.2)
+ *
+ * No PDF-specific translation engine exists; `source` is metadata only.
+ */
+
+import { getSettings, saveSettings } from './settings-service.js';
+import { TranslationSession } from './translation-session.js';
+import { chatStream, listModels } from './ollama-client.js';
+import { buildSystemPrompt } from '../shared/prompt.js';
+import { buildOptions, toApiThinking, sanitizeSettings, defaultSettings } from '../shared/settings-schema.js';
+import { BG_TO_CONTENT, MAX_SELECTION_LENGTH, ERROR_CODES } from '../shared/constants.js';
+
+const session = new TranslationSession();
+
+/** User-facing messages per error code (kept minimal; no stack traces). */
+const ERROR_MESSAGES = {
+  [ERROR_CODES.INVALID_REQUEST]: '要求が不正です。',
+  [ERROR_CODES.TEXT_EMPTY]: '選択文字列が空です。',
+  [ERROR_CODES.TEXT_TOO_LONG]: `選択文字列が長すぎます（上限 ${MAX_SELECTION_LENGTH} 文字）。`,
+  [ERROR_CODES.OLLAMA_UNAVAILABLE]: 'Ollamaに接続できません。Ollamaが起動しているか確認してください。',
+  [ERROR_CODES.MODEL_NOT_FOUND]: '指定されたモデルが見つかりません。設定画面でモデルを選び直してください。',
+  [ERROR_CODES.INVALID_SETTINGS]: '設定値が不正です。設定画面を確認してください。',
+  [ERROR_CODES.REQUEST_ABORTED]: '翻訳を中断しました。',
+  [ERROR_CODES.STREAM_ERROR]: '翻訳中にエラーが発生しました。',
+  [ERROR_CODES.UNKNOWN_ERROR]: '不明なエラーが発生しました。'
+};
+
+const NO_MODEL_MESSAGE = 'モデルが設定されていません。ツールバーアイコンまたは設定画面でモデルを選択してください。';
+
+/**
+ * Background translation state (Design.md §9.2). Shared by popup and
+ * menu-controller; the popup reads it via GET_POPUP_STATE.
+ * @type {{requestId: string|null, sourceText: string, translatedText: string,
+ *         status: string, model: string, error: string|null, startedAt: number|null,
+ *         source: string}}
+ */
+const state = {
+  requestId: null,
+  sourceText: '',
+  translatedText: '',
+  status: 'idle',
+  model: '',
+  error: null,
+  startedAt: null,
+  source: ''
+};
+
+/**
+ * @returns {object} a copy of the current translation state.
+ */
+export function getTranslationState() {
+  return { ...state, translatedText: state.translatedText };
+}
+
+/**
+ * Reset the shared state for a new request.
+ * @param {string} requestId
+ * @param {string} text validated source text
+ * @param {string} source 'content' | 'context-menu'
+ */
+function beginState(requestId, text, source) {
+  state.requestId = requestId;
+  state.sourceText = text;
+  state.translatedText = '';
+  state.status = 'starting';
+  state.model = '';
+  state.error = null;
+  state.startedAt = Date.now();
+  state.source = source;
+}
+
+/**
+ * Validate an untrusted selection string (content script or
+ * menus.selectionText - both are untrusted, Security.md §7.4).
+ * @param {*} text
+ * @returns {{ok: true, text: string}|{ok: false, code: string}}
+ */
+export function validateSelectionText(text) {
+  if (typeof text !== 'string') {
+    return { ok: false, code: ERROR_CODES.INVALID_REQUEST };
+  }
+  const trimmed = text.trim();
+  if (trimmed.length === 0) {
+    return { ok: false, code: ERROR_CODES.TEXT_EMPTY };
+  }
+  // Strip control characters except tab/newline/CR.
+  const cleaned = trimmed.replace(/[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F]/g, '');
+  if (cleaned.length === 0) {
+    return { ok: false, code: ERROR_CODES.TEXT_EMPTY };
+  }
+  if (cleaned.length > MAX_SELECTION_LENGTH) {
+    return { ok: false, code: ERROR_CODES.TEXT_TOO_LONG };
+  }
+  return { ok: true, text: cleaned };
+}
+
+/**
+ * @param {string} code
+ * @returns {string} user-facing message for the code
+ */
+export function messageForCode(code) {
+  return ERROR_MESSAGES[code] || ERROR_MESSAGES[ERROR_CODES.UNKNOWN_ERROR];
+}
+
+/**
+ * Ensure a model is configured before translating. On a fresh install the
+ * user may translate straight from a context menu without ever opening the
+ * settings UI, so storage.model would still be ''. In that case pick the
+ * first model reported by the running Ollama (best-effort, one attempt per
+ * runTranslation call) and persist it so the choice is visible in the UI.
+ * This mirrors the popup/options auto-select behavior so every path agrees.
+ * @param {object} browserApi injectable for tests
+ * @returns {Promise<{settings: object, error: string|null}>}
+ */
+async function ensureModelConfigured(browserApi) {
+  let settings = await getSettings(browserApi);
+  if (settings.model) {
+    return { settings, error: null };
+  }
+  try {
+    const models = await listModels(settings.ollamaHost);
+    if (models.length > 0) {
+      const merged = sanitizeSettings({ ...settings, model: models[0] });
+      await saveSettings(merged, browserApi);
+      settings = merged;
+      return { settings, error: null };
+    }
+    return { settings, error: NO_MODEL_MESSAGE };
+  } catch (error) {
+    // Ollama unreachable while listing models: report a clear error.
+    const code = (error && error.code) || ERROR_CODES.OLLAMA_UNAVAILABLE;
+    return { settings, error: messageForCode(code) };
+  }
+}
+
+/**
+ * Popup state subscribers. The popup connects a 'popup-state' port and
+ * receives the shared state after every change, so an open popup shows
+ * live streaming progress even for sink-less (context-menu) requests.
+ * @type {Set<object>}
+ */
+const popupPorts = new Set();
+
+/**
+ * Register a popup port for live state broadcasts.
+ * @param {object} port runtime.Port (name 'popup-state')
+ */
+export function addPopupPort(port) {
+  popupPorts.add(port);
+  try {
+    port.postMessage({ type: 'popup-state', state: getTranslationState() });
+  } catch {
+    // ignore
+  }
+}
+
+/**
+ * Unregister a popup port.
+ * @param {object} port
+ */
+export function removePopupPort(port) {
+  popupPorts.delete(port);
+}
+
+/**
+ * Broadcast the current state to every connected popup port.
+ */
+function broadcastState() {
+  for (const port of popupPorts) {
+    try {
+      port.postMessage({ type: 'popup-state', state: getTranslationState() });
+    } catch {
+      popupPorts.delete(port);
+    }
+  }
+}
+
+/**
+ * Run one translation and notify a sink with started/chunk/completed/error.
+ *
+ * The sink is an object with an optional postMessage(message) method. The
+ * content-script port is one sink; the popup state is updated internally and
+ * broadcast to connected popups so they can render live progress.
+ *
+ * @param {object} params
+ * @param {string} params.text validated source text
+ * @param {string} params.requestId
+ * @param {string} params.source 'content' | 'context-menu'
+ * @param {object|null} [params.sink] optional message sink (port-like)
+ * @returns {Promise<void>}
+ */
+export async function runTranslation({ text, requestId, source, sink = null, browserApi = null }) {
+  let settings = await getSettings(browserApi);
+
+  if (!settings.enabled) {
+    state.status = 'error';
+    state.error = messageForCode(ERROR_CODES.INVALID_REQUEST);
+    broadcastState();
+    postToSink(sink, { type: BG_TO_CONTENT.ERROR, requestId, code: ERROR_CODES.INVALID_REQUEST, message: state.error });
+    return;
+  }
+
+  // Fresh-install path: pick the first model from the running Ollama so a
+  // user who never opened the settings UI can still translate.
+  const ensured = await ensureModelConfigured(browserApi);
+  if (ensured.error) {
+    state.status = 'error';
+    state.error = ensured.error;
+    broadcastState();
+    postToSink(sink, { type: BG_TO_CONTENT.ERROR, requestId, code: ERROR_CODES.INVALID_SETTINGS, message: ensured.error });
+    return;
+  }
+  settings = ensured.settings;
+
+  // Registering the new request aborts any previous one.
+  const controller = session.start(requestId, sink);
+
+  beginState(requestId, text, source);
+  state.model = settings.model;
+  broadcastState();
+
+  postToSink(sink, { type: BG_TO_CONTENT.STARTED, requestId, model: settings.model });
+
+  try {
+    await chatStream({
+      host: settings.ollamaHost,
+      model: settings.model,
+      messages: [
+        { role: 'system', content: buildSystemPrompt(settings) },
+        { role: 'user', content: text }
+      ],
+      think: toApiThinking(settings),
+      options: buildOptions(settings),
+      signal: controller.signal,
+      onChunk: (content) => {
+        // Drop chunks if a newer request took over.
+        if (!session.isActive(requestId)) return;
+        if (content) {
+          state.translatedText += content;
+          state.status = 'streaming';
+          broadcastState();
+        }
+        postToSink(sink, { type: BG_TO_CONTENT.CHUNK, requestId, text: content });
+      }
+    });
+    if (session.isActive(requestId)) {
+      state.status = 'completed';
+      broadcastState();
+      postToSink(sink, { type: BG_TO_CONTENT.COMPLETED, requestId });
+    }
+  } catch (error) {
+    const code = (error && error.code) || ERROR_CODES.UNKNOWN_ERROR;
+    if (code === ERROR_CODES.REQUEST_ABORTED) {
+      // Reported by TranslationSession.abortActive() to the old sink; the
+      // shared state marks it cancelled so the popup can show it.
+      state.status = 'cancelled';
+      state.error = messageForCode(code);
+      broadcastState();
+    } else if (session.isActive(requestId)) {
+      state.status = 'error';
+      // Fall back to the code's base message when the detail is empty or
+      // judged non-user-facing by detailForUser().
+      const detail = detailForUser(error);
+      state.error = detail ? `${messageForCode(code)}\n(${detail})` : messageForCode(code);
+      broadcastState();
+      postToSink(sink, { type: BG_TO_CONTENT.ERROR, requestId, code, detail: state.error });
+    }
+  } finally {
+    session.clear(requestId);
+  }
+}
+
+/**
+ * Build a short, user-safe detail string from an error. Low-level exception
+ * text and full server responses are not shown unbounded: keep the message
+ * short and drop anything that looks like an internal URL or a huge blob.
+ * @param {*} error
+ * @returns {string} '' when there is nothing worth showing
+ */
+function detailForUser(error) {
+  const base = messageForCode(ERROR_CODES.UNKNOWN_ERROR);
+  const raw = error && error.message ? String(error.message) : '';
+  if (!raw) return '';
+  // Skip raw network errors that leak no useful info beyond the code.
+  if (/failed to fetch|networkerror|load failed/i.test(raw)) return '';
+  // Strip URLs (internal host names should not be echoed to the UI).
+  const cleaned = raw.replace(/\bhttps?:\/\/\S+/gi, '').trim();
+  if (!cleaned) return '';
+  // Drop obviously non-user-facing payloads (full JSON responses etc.).
+  if (cleaned.startsWith('{') || cleaned.startsWith('[')) return '';
+  const short = cleaned.length > 120 ? `${cleaned.slice(0, 120)}…` : cleaned;
+  return `${short}`;
+}
+
+/**
+ * @param {object|null} sink
+ * @param {object} message
+ */
+function postToSink(sink, message) {
+  if (!sink) return;
+  try {
+    sink.postMessage(message);
+  } catch {
+    // sink may be disconnected (e.g. popup closed)
+  }
+}
+
+/** @returns {TranslationSession} the shared session (for tests / cancel). */
+export function getSession() {
+  return session;
+}
+
+export { ERROR_MESSAGES, NO_MODEL_MESSAGE };
\ No newline at end of file
diff --git a/src/popup/popup.css b/src/popup/popup.css
index 964454d..de287a9 100644
--- a/src/popup/popup.css
+++ b/src/popup/popup.css
@@ -8,9 +8,80 @@ body {
   background: #ffffff;
 }
 
+.popup-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-bottom: 10px;
+}
+
 h1 {
   font-size: 14px;
-  margin: 0 0 10px;
+  margin: 0;
+}
+
+.mode-nav {
+  display: flex;
+  gap: 4px;
+}
+
+.nav-btn {
+  border: 1px solid rgba(0, 0, 0, 0.25);
+  background: #fff;
+  border-radius: 4px;
+  padding: 2px 10px;
+  cursor: pointer;
+  font-size: 12px;
+}
+
+.nav-btn.active {
+  background: #1a73e8;
+  border-color: #1a73e8;
+  color: #fff;
+}
+
+.translation-text {
+  min-height: 60px;
+  max-height: 220px;
+  overflow-y: auto;
+  white-space: pre-wrap;
+  word-break: break-word;
+  border: 1px solid rgba(0, 0, 0, 0.15);
+  border-radius: 4px;
+  padding: 6px 8px;
+  margin: 6px 0;
+  user-select: text;
+}
+
+.meta {
+  color: #5f6368;
+  font-size: 11px;
+  margin: 2px 0 0;
+}
+
+.actions {
+  display: flex;
+  gap: 6px;
+  margin-top: 8px;
+}
+
+.actions button {
+  flex: 1;
+  padding: 5px 8px;
+  border: 1px solid rgba(0, 0, 0, 0.25);
+  border-radius: 4px;
+  background: #fff;
+  cursor: pointer;
+}
+
+.actions button:hover:not([disabled]) {
+  background: #f0f4ff;
+}
+
+.actions button[disabled] {
+  opacity: 0.5;
+  cursor: default;
 }
 
 .row {
diff --git a/src/popup/popup.html b/src/popup/popup.html
index e51475f..06a4cf4 100644
--- a/src/popup/popup.html
+++ b/src/popup/popup.html
@@ -6,28 +6,50 @@
 </head>
 <body>
   <main>
-    <h1>Ollama Translations</h1>
+    <header class="popup-header">
+      <h1>Ollama Translations</h1>
+      <nav class="mode-nav" aria-label="表示切替">
+        <button id="nav-settings" type="button" class="nav-btn active" aria-pressed="true">設定</button>
+        <button id="nav-translation" type="button" class="nav-btn" aria-pressed="false">翻訳</button>
+      </nav>
+    </header>
 
-    <section class="row toggle-row">
-      <label class="toggle-label" for="enabled-toggle">翻訳機能</label>
-      <input type="checkbox" id="enabled-toggle" role="switch">
-    </section>
-
-    <section class="row">
-      <label for="model-select">モデル</label>
-      <div class="model-row">
-        <select id="model-select"></select>
-        <button id="reload-models" type="button" title="モデル一覧を再取得">↻</button>
+    <!-- Translation result mode -->
+    <section id="translation-mode" class="mode" hidden>
+      <p id="translation-status" class="status"></p>
+      <p id="translation-meta" class="meta" hidden></p>
+      <div id="translation-text" class="translation-text" aria-live="polite"></div>
+      <p id="translation-error" class="status error" hidden></p>
+      <div class="actions">
+        <button id="copy-translation" type="button" disabled>コピー</button>
+        <button id="regenerate-translation" type="button" disabled>再生成</button>
+        <button id="open-options-from-translation" type="button">設定を開く</button>
       </div>
-      <p id="model-status" class="status" hidden></p>
     </section>
 
-    <section class="row">
-      <label for="language-select">翻訳先言語</label>
-      <select id="language-select"></select>
-    </section>
+    <!-- Settings mode -->
+    <section id="settings-mode" class="mode">
+      <section class="row toggle-row">
+        <label class="toggle-label" for="enabled-toggle">翻訳機能</label>
+        <input type="checkbox" id="enabled-toggle" role="switch">
+      </section>
 
-    <button id="open-options" type="button">詳細設定を開く</button>
+      <section class="row">
+        <label for="model-select">モデル</label>
+        <div class="model-row">
+          <select id="model-select"></select>
+          <button id="reload-models" type="button" title="モデル一覧を再取得">↻</button>
+        </div>
+        <p id="model-status" class="status" hidden></p>
+      </section>
+
+      <section class="row">
+        <label for="language-select">翻訳先言語</label>
+        <select id="language-select"></select>
+      </section>
+
+      <button id="open-options" type="button">詳細設定を開く</button>
+    </section>
   </main>
   <script type="module" src="popup.js"></script>
 </body>
diff --git a/src/popup/popup.js b/src/popup/popup.js
index eb213f5..3bd0cd9 100644
--- a/src/popup/popup.js
+++ b/src/popup/popup.js
@@ -1,6 +1,11 @@
 'use strict';
 
-import { LANGUAGES, ERROR_CODES, RUNTIME_MESSAGES } from '../shared/constants.js';
+import {
+  LANGUAGES,
+  ERROR_CODES,
+  RUNTIME_MESSAGES,
+  POPUP_STATUSES
+} from '../shared/constants.js';
 import { defaultSettings, sanitizeSettings } from '../shared/settings-schema.js';
 
 const $enabled = /** @type {HTMLInputElement} */ (document.getElementById('enabled-toggle'));
@@ -10,9 +15,171 @@ const $modelStatus = /** @type {HTMLParagraphElement} */ (document.getElementByI
 const $language = /** @type {HTMLSelectElement} */ (document.getElementById('language-select'));
 const $openOptions = /** @type {HTMLButtonElement} */ (document.getElementById('open-options'));
 
+// Mode navigation + translation result mode elements
+const $navSettings = /** @type {HTMLButtonElement} */ (document.getElementById('nav-settings'));
+const $navTranslation = /** @type {HTMLButtonElement} */ (document.getElementById('nav-translation'));
+const $settingsMode = /** @type {HTMLElement} */ (document.getElementById('settings-mode'));
+const $translationMode = /** @type {HTMLElement} */ (document.getElementById('translation-mode'));
+const $translationStatus = /** @type {HTMLParagraphElement} */ (document.getElementById('translation-status'));
+const $translationMeta = /** @type {HTMLParagraphElement} */ (document.getElementById('translation-meta'));
+const $translationText = /** @type {HTMLDivElement} */ (document.getElementById('translation-text'));
+const $translationError = /** @type {HTMLParagraphElement} */ (document.getElementById('translation-error'));
+const $copyTranslation = /** @type {HTMLButtonElement} */ (document.getElementById('copy-translation'));
+const $regenerateTranslation = /** @type {HTMLButtonElement} */ (document.getElementById('regenerate-translation'));
+const $openOptionsFromTranslation = /** @type {HTMLButtonElement} */ (document.getElementById('open-options-from-translation'));
+
 /** @type {string[]} current model list for change detection */
 let knownModels = [];
 
+// ---------------------------------------------------------------------
+// Mode switching
+// ---------------------------------------------------------------------
+
+/** @param {'settings'|'translation'} mode */
+function showMode(mode) {
+  const translation = mode === 'translation';
+  $settingsMode.hidden = translation;
+  $translationMode.hidden = !translation;
+  $navSettings.classList.toggle('active', !translation);
+  $navTranslation.classList.toggle('active', translation);
+  $navSettings.setAttribute('aria-pressed', String(!translation));
+  $navTranslation.setAttribute('aria-pressed', String(translation));
+}
+
+$navSettings.addEventListener('click', () => {
+  showMode('settings');
+  void persistMode('settings');
+});
+$navTranslation.addEventListener('click', () => {
+  showMode('translation');
+  void persistMode('translation');
+  void refreshTranslationState();
+});
+
+$openOptionsFromTranslation.addEventListener('click', () => {
+  browser.runtime.openOptionsPage();
+});
+
+/**
+ * Remember the tab the user picked so the next popup open restores it
+ * instead of always jumping to the translation tab.
+ * @param {'settings'|'translation'} mode
+ */
+async function persistMode(mode) {
+  try {
+    await browser.storage.local.set({ popupMode: mode });
+  } catch {
+    // non-fatal
+  }
+}
+
+// ---------------------------------------------------------------------
+// Translation result mode
+// ---------------------------------------------------------------------
+
+const STATUS_LABELS = {
+  [POPUP_STATUSES.IDLE]: '待機中',
+  [POPUP_STATUSES.STARTING]: '翻訳を準備中…',
+  [POPUP_STATUSES.STREAMING]: '受信中…',
+  [POPUP_STATUSES.COMPLETED]: '完了',
+  [POPUP_STATUSES.ERROR]: 'エラー',
+  [POPUP_STATUSES.CANCELLED]: '中断'
+};
+
+function renderTranslationState(state) {
+  if (!state || typeof state !== 'object') return;
+  const status = typeof state.status === 'string' ? state.status : POPUP_STATUSES.IDLE;
+  const busy = status === POPUP_STATUSES.STARTING || status === POPUP_STATUSES.STREAMING;
+
+  $translationStatus.textContent = STATUS_LABELS[status] || status;
+
+  if (state.model) {
+    $translationMeta.hidden = false;
+    $translationMeta.textContent = `Ollama / ${state.model}`;
+  } else {
+    $translationMeta.hidden = true;
+  }
+
+  // Plain-text rendering only (Security.md §7.4).
+  $translationText.textContent = typeof state.translatedText === 'string' ? state.translatedText : '';
+
+  if (state.error) {
+    $translationError.hidden = false;
+    $translationError.textContent = state.error;
+  } else {
+    $translationError.hidden = true;
+  }
+
+  $copyTranslation.disabled = busy || !(state.translatedText && state.translatedText.length > 0);
+  $regenerateTranslation.disabled = busy || !state.sourceText;
+}
+
+async function refreshTranslationState() {
+  try {
+    const response = await browser.runtime.sendMessage({ type: RUNTIME_MESSAGES.GET_POPUP_STATE });
+    if (response && response.ok && response.state) {
+      renderTranslationState(response.state);
+      return response.state;
+    }
+  } catch {
+    // background unavailable; leave the current view
+  }
+  return null;
+}
+
+// ---------------------------------------------------------------------
+// Live state updates: subscribe to the shared background state so an
+// open popup receives streaming chunks in real time (Design.md §7.4).
+// ---------------------------------------------------------------------
+
+try {
+  const statePort = browser.runtime.connect({ name: 'popup-state' });
+  statePort.onMessage.addListener((message) => {
+    if (message && message.type === 'popup-state' && message.state) {
+      renderTranslationState(message.state);
+    }
+  });
+  // onDisconnect: popup is closing or the background restarted; nothing to do.
+} catch {
+  // Port may be unavailable in non-extension test contexts; polling in
+  // refreshTranslationState() still covers the initial state.
+}
+
+$copyTranslation.addEventListener('click', () => {
+  const text = $translationText.textContent || '';
+  if (!text) return;
+  void (async () => {
+    try {
+      await navigator.clipboard.writeText(text);
+      $translationStatus.textContent = 'コピーしました';
+    } catch {
+      // No execCommand fallback (deprecated): show failure only.
+      $translationStatus.textContent = 'コピーできませんでした';
+    }
+  })();
+});
+
+$regenerateTranslation.addEventListener('click', () => {
+  void (async () => {
+    try {
+      const response = await browser.runtime.sendMessage({ type: RUNTIME_MESSAGES.REGENERATE_LAST });
+      if (response && response.ok) {
+        await refreshTranslationState();
+      } else {
+        $translationError.hidden = false;
+        $translationError.textContent = '再生成を開始できませんでした。';
+      }
+    } catch {
+      $translationError.hidden = false;
+      $translationError.textContent = '再生成を開始できませんでした。';
+    }
+  })();
+});
+
+// ---------------------------------------------------------------------
+// Settings mode (existing behavior)
+// ---------------------------------------------------------------------
+
 function fillLanguages(selected) {
   $language.textContent = '';
   for (const lang of LANGUAGES) {
@@ -138,10 +305,23 @@ void (async () => {
   // If the model list loaded and no model is saved yet, persist the
   // auto-selected first model so the stored settings match the UI.
   if (knownModels.length > 0) {
-    const result = await browser.storage.local.get({ settings: null });
+    const result = await browser.storage.local.get({ settings: null, popupMode: null });
     const stored = sanitizeSettings(result.settings || defaultSettings());
     if (!stored.model) {
       await persist({ model: $model.value });
     }
+    // Decide the initial mode. Active/recent translation wins; otherwise
+    // restore the tab the user last picked (default: settings).
+    const state = await refreshTranslationState();
+    const active = state && (state.status === 'starting' || state.status === 'streaming');
+    if (active) {
+      showMode('translation');
+    } else if (result.popupMode === 'translation' && state && state.translatedText && state.translatedText.length > 0) {
+      showMode('translation');
+    } else {
+      showMode('settings');
+    }
+  } else {
+    showMode('settings');
   }
 })();
\ No newline at end of file
diff --git a/src/shared/constants.js b/src/shared/constants.js
index 76a679c..adae26d 100644
--- a/src/shared/constants.js
+++ b/src/shared/constants.js
@@ -67,5 +67,21 @@ export const BG_TO_CONTENT = {
 
 /** Message types: one-shot runtime messages (popup/options -> background). */
 export const RUNTIME_MESSAGES = {
-  LIST_MODELS: 'listModels'
+  LIST_MODELS: 'listModels',
+  GET_POPUP_STATE: 'getPopupState',
+  START_CONTEXT_MENU_TRANSLATION: 'startContextMenuTranslation',
+  REGENERATE_LAST: 'regenerateLast'
+};
+
+/** Context menu item id (menus API). */
+export const MENU_ID_TRANSLATE_SELECTION = 'translate-selection';
+
+/** Popup translation result statuses (Design.md §7.4). */
+export const POPUP_STATUSES = {
+  IDLE: 'idle',
+  STARTING: 'starting',
+  STREAMING: 'streaming',
+  COMPLETED: 'completed',
+  ERROR: 'error',
+  CANCELLED: 'cancelled'
 };
\ No newline at end of file