Offline tab as Files view, automatic background sync, sync notifications

- Offline tab is now FilesView(offline: true) over a FolderBrowser interface
  (FilesController / OfflineController) sharing controls, tiles, thumbnails
- Synced files follow the Files Cache rule (default: refresh every 15 min):
  per-account WorkManager jobs, in-app timer, resume and pull triggers;
  root-etag shortcut skips full walks when nothing changed
- Single sync at a time (shared lock); logging out stops that account's sync
- Sync safety: PROPFIND failures skip the path instead of deleting local files
- Mirror empty folders and remove deleted ones; drop synced paths deleted on
  the server; missing local files are re-downloaded
- Notifications: silent per-account sync notifications, "Background sync
  notifications" setting, audible upload/download completion
- Files refresh in place (no spinner flash); retry after network returns
- Fix sync badges not updating (shared native status stream), "Sync off" on
  launch (eager providers), Files Cache section moved under Device Sync

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.claude/context/standards.md

@@ -62,9 +62,13 @@ class/method already makes obvious.
 ## Testing
 
 - Widget tests must mock platform channels that the app touches on startup
-  — `SharedPreferences.setMockInitialValues({})` and a mock
-  `MethodChannel('plugins.it_nomads.com/flutter_secure_storage')` handler —
-  and disable Google Fonts network fetching
+  — `SharedPreferences.setMockInitialValues({})`, a mock
+  `MethodChannel('plugins.it_nomads.com/flutter_secure_storage')` handler,
+  and `connectivity_plus`'s `MethodChannel('dev.fluttercommunity.plus/
+  connectivity')` (`'check'` → a result list, e.g. `['wifi']`) plus its
+  `EventChannel('dev.fluttercommunity.plus/connectivity_status')` (a
+  `MockStreamHandler.inline` with a no-op `onListen` is enough) — and
+  disable Google Fonts network fetching
   (`GoogleFonts.config.allowRuntimeFetching = false`) in `setUpAll`. Without
   these, `SessionController`'s session restore never resolves in the test
   sandbox (no plugin implementation is registered, so the read future just