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/lib/storage.js

@@ -25,6 +25,7 @@ const DEFAULT_SETTINGS = {
   maxLogEntries: 200,
   customDomains: [],
   categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'domain', 'general'],
+  browserSync: false,
 };
 
 const Storage = {
@@ -36,7 +37,7 @@ const Storage = {
   },
 
   async saveMappings(mappings) {
-    await api.storage.local.set({ [KEYS.MAPPINGS]: mappings });
+    await api.storage.local.set({ [KEYS.MAPPINGS]: mappings, ss_lastModified: Date.now() });
   },
 
   async addMapping(mapping) {
@@ -96,7 +97,7 @@ const Storage = {
   },
 
   async saveProfiles(profiles) {
-    await api.storage.local.set({ [KEYS.IDENTITY]: { profiles } });
+    await api.storage.local.set({ [KEYS.IDENTITY]: { profiles }, ss_lastModified: Date.now() });
   },
 
   async addProfile(name) {
@@ -210,6 +211,7 @@ const Storage = {
     const current = await this.getSettings();
     await api.storage.local.set({
       [KEYS.SETTINGS]: { ...current, ...settings },
+      ss_lastModified: Date.now(),
     });
   },
 };