Dedupe native transfer/notification code and split ServerProvider into focused controllers

Consolidates duplicated GET/PUT/notification-channel logic across
DownloadService/ShareUploadService/SyncEngine into shared Kotlin helpers,
gives upload/download real batch queueing instead of dropping a second
concurrent batch, and dedupes repeated Dart channel-argument boilerplate.

Replaces the 2300+ line ServerProvider god object with ten focused
ChangeNotifiers (SessionController, SettingsController, FilesController,
PhotosController, FavoritesController, TrashController, SharesController,
RecentController, SyncStatusController, PickController) plus ItemOperations,
a plain coordinator for cross-domain item mutations - fixing the coupling
where device-sync status, per-tab data, and global UI prefs all lived in
one object. Updates every view/widget call site accordingly and refreshes
the architecture/server/standards/styling docs to match.

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

lib/models/app_tab.dart

@@ -3,7 +3,7 @@ import 'package:material_symbols_icons/symbols.dart';
 
 /// Every destination the bottom nav bar can show. Order here is only the
 /// fallback default — actual order/visibility/default-tab are user
-/// configurable and persisted in [ServerProvider].
+/// configurable and persisted in `SettingsController`.
 enum AppTab { files, photos, favorites, activity, trash, shares, recent }
 
 /// At most this many tabs may be visible in the bottom nav bar at once —