fix: diagnosable translation errors and icon hidden after scroll

Translation error ("設定値が不正です") diagnosis
- The INVALID_SETTINGS code collapsed two unrelated failures into one
  opaque message: (a) no model configured in the background settings
  cache, and (b) HTTP 400/403 from Ollama with the actual reason in the
  response body. Verified against a live Ollama server: the configured
  cloud models (glm-5.3:cloud etc.) stream fine from a plain page, so
  the generic message gave no clue which path fired.
- ollama-client.js: read the Ollama error body on non-OK responses and
  append its message ("model is required" etc.) to the thrown error
- message-router.js: no-model-configured now has its own explicit
  message ("モデルが設定されていません...") and concrete detail reasons
  are appended to the base message in the popover, e.g.
  設定値が不正です。... (Ollama /api/chat failed: 400 - ...)

Action icon disappearing
- The scroll handler hid the action button whenever the popover was
  hidden, even with a live selection, leaving no way to translate until
  the user re-selected. It now repositions the icon while a selection
  exists and only hides it when the selection is gone (Design 5.4).

Results: 73 passed, 0 failed. dist rebuilt.

2 files changed, 36 insertions, 7 deletions

diff --git a/src/background/message-router.js b/src/background/message-router.js
index c93fc9d..9428312 100644
--- a/src/background/message-router.js
+++ b/src/background/message-router.js
@@ -31,13 +31,16 @@ const ERROR_MESSAGES = {
   [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。',
+  [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。';
+
 /**
  * Handle one content-script port connection.
  * @param {object} port runtime.Port
@@ -84,7 +87,7 @@ async function runTranslation(port, requestId, text) {
     return;
   }
   if (!settings.model) {
-    postError(port, requestId, ERROR_CODES.INVALID_SETTINGS, 'No model configured');
+    postError(port, requestId, ERROR_CODES.INVALID_SETTINGS, NO_MODEL_MESSAGE);
     return;
   }
 
@@ -128,11 +131,15 @@ async function runTranslation(port, requestId, text) {
  * @param {object} port
  * @param {string} requestId
  * @param {string} code
- * @param {string} message
+ * @param {string} detail optional concrete reason shown to the user
  */
-function postError(port, requestId, code, message) {
+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: ERROR_MESSAGES[code] || ERROR_MESSAGES[ERROR_CODES.UNKNOWN_ERROR] });
+    port.postMessage({ type: BG_TO_CONTENT.ERROR, requestId, code, message });
   } catch {
     // port may be disconnected
   }
diff --git a/src/background/ollama-client.js b/src/background/ollama-client.js
index 0286230..e53d997 100644
--- a/src/background/ollama-client.js
+++ b/src/background/ollama-client.js
@@ -16,6 +16,24 @@ import { isValidOllamaHost } from '../shared/settings-schema.js';
 import { ERROR_CODES } from '../shared/constants.js';
 
 /**
+ * Extract a short error message from an Ollama JSON error body.
+ * @param {string} raw
+ * @returns {string} message or '' when not parseable
+ */
+function extractOllamaError(raw) {
+  if (!raw) return '';
+  try {
+    const parsed = JSON.parse(raw);
+    if (parsed && typeof parsed.error === 'string') {
+      return parsed.error.slice(0, 200);
+    }
+  } catch {
+    // not JSON
+  }
+  return '';
+}
+
+/**
  * Translate a fetch/HTTP failure into a stable error code.
  * @param {Response|undefined} response
  * @returns {string} error code
@@ -73,7 +91,9 @@ export async function listModels(host, signal) {
     throw toExtensionError(error);
   }
   if (!response.ok) {
-    const e = new Error(`Ollama /api/tags failed: ${response.status}`);
+    const detail = await response.text().catch(() => '');
+    const serverMessage = extractOllamaError(detail);
+    const e = new Error(`Ollama /api/tags failed: ${response.status}${serverMessage ? ` - ${serverMessage}` : ''}`);
     e.code = response.status === 404 ? ERROR_CODES.MODEL_NOT_FOUND : ERROR_CODES.STREAM_ERROR;
     throw e;
   }
@@ -119,7 +139,9 @@ export async function chatStream({ host, model, messages, think, options, onChun
   }
 
   if (!response.ok) {
-    const e = new Error(`Ollama /api/chat failed: ${response.status}`);
+    const detail = await response.text().catch(() => '');
+    const serverMessage = extractOllamaError(detail);
+    const e = new Error(`Ollama /api/chat failed: ${response.status}${serverMessage ? ` - ${serverMessage}` : ''}`);
     e.code =
       response.status === 404
         ? ERROR_CODES.MODEL_NOT_FOUND