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>

lib/widgets/app_tab_view_builder.dart

@@ -14,7 +14,12 @@ import '../views/trash_view.dart';
 Widget buildAppTabView(AppTab tab, ScrollController controller) {
   switch (tab) {
     case AppTab.files:
-      return FilesView(scrollController: controller);
+      // Keyed so the Files and Offline tabs (same widget type) never share
+      // State when the visible-tab list shifts positions in the IndexedStack.
+      return FilesView(
+        key: const ValueKey('files'),
+        scrollController: controller,
+      );
     case AppTab.photos:
       return PhotosView(scrollController: controller);
     case AppTab.favorites:
@@ -27,5 +32,11 @@ Widget buildAppTabView(AppTab tab, ScrollController controller) {
       return SharesView(scrollController: controller);
     case AppTab.recent:
       return RecentView(scrollController: controller);
+    case AppTab.offline:
+      return FilesView(
+        key: const ValueKey('offline'),
+        scrollController: controller,
+        offline: true,
+      );
   }
 }