feat: Firefox selection translation extension with local Ollama (initial version)

Implement the complete initial version of a Firefox WebExtension (Manifest V3)
that translates selected text on web pages using a local Ollama LLM and shows
the result in a small popover near the selection.

Specification alignment (Phase 0)
- Unified Specifications.md / Design.md / Security.md on the same approach:
  Ollama REST API + Fetch API + NDJSON streaming (no ollama-js SDK dependency)
- Fixed host_permissions to http://localhost/* and http://127.0.0.1/* because
  Firefox match patterns do not support port numbers (bug 1362809/1468162);
  the actual connection target is restricted to localhost/127.0.0.1 + any port
  by application-side validation (defense in depth, OLL-09)
- Clipboard: use navigator.clipboard.writeText() only inside the copy button
  click (user activation); no deprecated execCommand fallback

Implementation (zero dependencies, vanilla JS, no npm/Node required)
- manifest.json: MV3, background.scripts + type:module (event page, no
  service_worker), action popup, options_ui, content_scripts for http/https,
  permissions: storage + localhost host permissions only
- src/shared: constants (ports, limits, presets, error codes), message
  validation, settings schema (defaults / merge / sanitize / buildOptions /
  toApiThinking), system prompt builder (fixed prompt first + user prompt)
- src/background: message-router (runtime.Port streaming + runtime.onMessage
  for model list), translation-session (single active request, abort previous
  on new request, stale chunk suppression), ollama-client (fetch against
  /api/tags and /api/chat with buffering NDJSON line parser tolerant to chunk
  boundaries inside JSON objects, AbortController, host re-validation before
  every request, HTTP status to error-code mapping), settings-service
  (storage.local load/save with validation and fallback to defaults)
- src/content/content.js: single classic script (Firefox does not support ES
  modules in content scripts) with selection detection (selectionchange +
  mouseup + keyup, debounced), Shadow DOM action button, Shadow DOM popover
  with HIDDEN/LOADING/STREAMING/COMPLETED/ERROR state machine, port client
  with requestId-based stale chunk filtering, enabled-state sync via
  storage.onChanged; all dynamic text rendered via textContent only
- src/popup + src/options: full settings UI (enable toggle, Ollama host,
  model list from Ollama with saved-model warning, target language, thinking
  mode, temperature slider 0.0-2.0 step 0.1, context length presets
  4k/8k/16k/32k/128k/256k, seed random/fixed with integer value, user system
  prompt, advanced numeric options with steppers, stop sequences textarea)
- icons generated by tools/make-icons.ps1 (System.Drawing, no external tools)
- build.ps1 packages manifest.json + icons/ + src/ into dist zip, excluding
  tests/, doc/, ollama-js/ and tools/

Tests (71 passed, 0 failed)
- In-browser test runner (tests/test.html) with custom describe/it/assert
  framework; no npm, no Node.js
- Unit: constants sync between content mirror and shared module, Ollama host
  validation (localhost/127.0.0.1 + any port only; rejects external hosts,
  LAN IPs, https, credentials, paths), settings schema defaults and fallback,
  temperature 0.1-step validation, context length presets, seed modes,
  advanced option types, buildOptions omission of unset values and absence of
  tfs_z/mirostat fields, system prompt composition order, message validation
  with selection length boundary values
- Integration: NDJSON streaming across awkward byte boundaries, thinking
  chunk forwarding, malformed line tolerance, error payload mapping, think/
  options inclusion rules, model list, TranslationSession abort semantics,
  settings round-trip with storage.onChanged cache refresh, full port flow
  (started/chunk/completed), newer request aborts older with stale chunk
  discard, malformed message rejection, OLLAMA_UNAVAILABLE and
  INVALID_SETTINGS error paths, untrusted sender rejection

Security audit (Design.md section 24 checklist: all pass)
- No innerHTML/outerHTML/document.write/eval/new Function/localStorage
- No remote JS/CSS/fonts/CDN resources; no inline scripts in extension pages
- Ollama access limited to background; content scripts never fetch Ollama
- Background validates message type/length/requestId; single concurrent
  translation enforced
- Clipboard API only (no execCommand fallback)
- Minimal permissions: storage + localhost host permissions

Docs
- README.md with build/load/usage instructions, API list, permission rationale
- doc/Testing-Results.md with automated test results, security audit results,
  and a 14-item manual Firefox verification checklist (to be executed on a
  real Firefox + Ollama environment)

1 files changed, 69 insertions, 0 deletions

diff --git a/src/shared/messages.js b/src/shared/messages.js
new file mode 100644
index 0000000..95a257e
--- /dev/null
+++ b/src/shared/messages.js
@@ -0,0 +1,69 @@
+/**
+ * Message validation helpers (background side).
+ *
+ * All messages received from content scripts must pass through these
+ * validators before they are used. The page origin is untrusted
+ * (Security.md §7): every field is type-checked and length-limited.
+ */
+
+import { CONTENT_TO_BG, MAX_SELECTION_LENGTH } from './constants.js';
+
+/**
+ * Generate a random request id. Uses crypto.randomUUID when available and
+ * falls back to a random-hex string otherwise.
+ * @returns {string}
+ */
+export function createRequestId() {
+  if (typeof crypto !== 'undefined' && typeof crypto.randomUUID === 'function') {
+    return crypto.randomUUID();
+  }
+  const bytes = new Uint8Array(16);
+  if (typeof crypto !== 'undefined' && typeof crypto.getRandomValues === 'function') {
+    crypto.getRandomValues(bytes);
+  } else {
+    for (let i = 0; i < bytes.length; i++) bytes[i] = Math.floor(Math.random() * 256);
+  }
+  return Array.from(bytes, (b) => b.toString(16).padStart(2, '0')).join('');
+}
+
+/**
+ * @param {*} message
+ * @returns {boolean} true when the message is a valid translate request.
+ */
+export function isValidTranslateRequest(message) {
+  if (message === null || typeof message !== 'object' || Array.isArray(message)) {
+    return false;
+  }
+  if (message.type !== CONTENT_TO_BG.TRANSLATE) return false;
+  if (typeof message.requestId !== 'string' || message.requestId.length === 0 || message.requestId.length > 128) {
+    return false;
+  }
+  if (typeof message.text !== 'string') return false;
+  if (message.text.length === 0) return false;
+  if (message.text.length > MAX_SELECTION_LENGTH) return false;
+  return true;
+}
+
+/**
+ * @param {*} message
+ * @returns {boolean} true when the message is a valid cancel request.
+ */
+export function isValidCancelRequest(message) {
+  if (message === null || typeof message !== 'object' || Array.isArray(message)) {
+    return false;
+  }
+  if (message.type !== CONTENT_TO_BG.CANCEL) return false;
+  return typeof message.requestId === 'string' && message.requestId.length > 0 && message.requestId.length <= 128;
+}
+
+/**
+ * Check whether the sender is a background-trusted extension context
+ * (i.e. a message coming from our own content script, popup or options).
+ * @param {unknown} sender WebExtensions MessageSender
+ * @returns {boolean}
+ */
+export function isTrustedSender(sender) {
+  // Only extension-internal senders carry an id; external senders are not
+  // accepted anyway because onMessageExternal is not used.
+  return Boolean(sender && sender.id);
+}
\ No newline at end of file