fix: date false-positive, partial-word highlights, cross-browser sync

Bug fixes:
- Date (possible DOB) pattern now requires context words (born, birthday,
  dob, etc.) before firing — prevents spurious warnings on page-load API
  calls that happen to contain ISO dates in conversation history.
- Highlight regex now uses word boundaries (\b) so short substitute values
  (e.g. "aud") no longer match inside unrelated words like "Claude".
- Both TreeWalkers in content.js now skip the extension's own UI elements
  (.ss-autodetect-warning, .ss-presend-warning, .ss-reveal-badge) to
  prevent the highlight API from marking text in the extension's banners.

Settings sync:
- New src/lib/sync.js: exportSyncCode / importSyncCode (base64 JSON) for
  manual copy-paste across any browser combination. Newest lastModified
  timestamp wins; force flag available to override.
- browser.storage.sync support: when "Browser account sync" is enabled the
  extension automatically pushes/pulls via Firefox Sync or Chrome account,
  chunked to stay within per-item quota limits.
- storage.js now writes ss_lastModified on every save so conflict resolution
  has an accurate timestamp.
- service-worker.js listens for both local and sync storage changes to keep
  all copies in sync.
- New "Sync Between Browsers" section in options.html with Generate/Copy/
  Import Sync Code UI and the browser sync toggle.

https://claude.ai/code/session_01TKpSR9M8JgHLXCp5CeDsQP

src/background/service-worker.js

@@ -7,6 +7,7 @@
  */
 
 import Storage from '../lib/storage.js';
+import SilentSendSync from '../lib/sync.js';
 import api from '../lib/browser-polyfill.js';
 
 // Track substitution counts per tab
@@ -296,11 +297,24 @@ async function updateIcon(settings) {
   }
 }
 
-// Update icon when settings, identity, or mappings change
-api.storage.onChanged.addListener(async (changes) => {
+// Update icon when settings, identity, or mappings change; push to sync storage if enabled
+api.storage.onChanged.addListener(async (changes, areaName) => {
   if (changes.ss_settings || changes.ss_identity || changes.ss_mappings) {
     const settings = await Storage.getSettings();
     await updateIcon(settings);
+
+    // Push to browser.storage.sync when local data changes (same-browser cross-device)
+    if (areaName === 'local' && settings.browserSync) {
+      await SilentSendSync.pushToSyncStorage();
+    }
+  }
+
+  // When sync storage changes (another device pushed new data), pull it into local
+  if (areaName === 'sync' && (changes.ss_sync_meta || Object.keys(changes).some(k => k.startsWith('ss_sync_chunk_')))) {
+    const settings = await Storage.getSettings();
+    if (settings.browserSync) {
+      await SilentSendSync.pullFromSyncStorage();
+    }
   }
 });
 
@@ -311,8 +325,11 @@ api.runtime.onInstalled.addListener(async () => {
   await updateIcon(settings);
 });
 
-// Also set icon on startup (service worker wake)
+// Also set icon on startup (service worker wake) + pull any newer sync data
 (async () => {
   const settings = await Storage.getSettings();
   await updateIcon(settings);
+  if (settings.browserSync) {
+    await SilentSendSync.pullFromSyncStorage();
+  }
 })();