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, 82 insertions, 0 deletions

diff --git a/src/background/settings-service.js b/src/background/settings-service.js
new file mode 100644
index 0000000..2687921
--- /dev/null
+++ b/src/background/settings-service.js
@@ -0,0 +1,82 @@
+/**
+ * Settings service (background): storage access with validation.
+ *
+ * Load flow (Design.md §17.2):
+ *   1. generate default settings
+ *   2. read storage
+ *   3. merge
+ *   4. validate
+ *   5. invalid values fall back to defaults
+ *
+ * The validated cache is refreshed on storage.onChanged.
+ */
+
+import { sanitizeSettings, defaultSettings } from '../shared/settings-schema.js';
+
+const STORAGE_KEY = 'settings';
+
+/** @type {object|null} cached validated settings */
+let cache = null;
+
+/**
+ * @param {object} [browserApi] injectable browser API (defaults to globalThis.browser)
+ * @returns {object}
+ */
+function api(browserApi) {
+  return browserApi || globalThis.browser;
+}
+
+/**
+ * Load settings from storage.local, sanitize and cache them.
+ * @param {object} [browserApi]
+ * @returns {Promise<object>} validated settings
+ */
+export async function loadSettings(browserApi) {
+  const b = api(browserApi);
+  let stored = null;
+  try {
+    const result = await b.storage.local.get({ settings: null });
+    stored = result.settings;
+  } catch {
+    stored = null;
+  }
+  cache = sanitizeSettings(stored);
+  return cache;
+}
+
+/**
+ * Save settings. Values are sanitized before writing.
+ * @param {object} settings raw settings object from the UI
+ * @param {object} [browserApi]
+ * @returns {Promise<object>} the validated settings actually stored
+ */
+export async function saveSettings(settings, browserApi) {
+  const b = api(browserApi);
+  const validated = sanitizeSettings(settings);
+  await b.storage.local.set({ settings: validated });
+  cache = validated;
+  return validated;
+}
+
+/**
+ * @param {object} [browserApi]
+ * @returns {Promise<object>} current validated settings (cached when possible)
+ */
+export async function getSettings(browserApi) {
+  if (cache) return cache;
+  return loadSettings(browserApi);
+}
+
+/**
+ * Register the storage.onChanged listener that keeps the cache in sync.
+ * Must be called once from the background entry point.
+ * @param {object} [browserApi]
+ */
+export function watchSettings(browserApi) {
+  const b = api(browserApi);
+  b.storage.onChanged.addListener((changes, areaName) => {
+    if (areaName === 'local' && changes.settings) {
+      cache = sanitizeSettings(changes.settings.newValue);
+    }
+  });
+}
\ No newline at end of file