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>

android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,10 @@
     <uses-permission android:name="android.permission.INTERNET"/>
     <!-- For local_auth's biometric app-lock (Settings > Security). -->
     <uses-permission android:name="android.permission.USE_BIOMETRIC"/>
+    <!-- For connectivity_plus (ConnectivityController), which gates the
+         Offline tab restriction and skips network requests entirely while
+         offline - see session_controller.dart/main.dart. -->
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <!-- For ShareUploadService/DownloadService, which keep a "Share to Noo"
          upload/download going (and cancellable from its notification) even
          if the app is fully closed - see each service's own doc comment. -->