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/sync_status.dart

@@ -34,5 +34,5 @@ enum SyncItemStatus { none, syncing, synced, conflict }
 
 /// The persistent header chip/panel's overall status (account-wide, not
 /// scoped to whatever folder is currently browsed) - see
-/// `ServerProvider.syncHeaderStatus`.
+/// `SyncStatusController.syncHeaderStatus`.
 enum SyncHeaderStatus { off, syncing, done, alert }