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/widgets/move_copy_conflict_sheet.dart

@@ -7,7 +7,7 @@ import 'gradual_bottom_sheet.dart';
 /// destination) - summarizes every conflict at once rather than prompting
 /// per item as they're hit, and lets the user resolve them all the same
 /// way (overwrite/keep both) or open a per-item breakdown. Returns the
-/// `ConflictChoice` map `ServerProvider.resolveConflicts` expects, or null
+/// `ConflictChoice` map `ItemOperations.resolveConflicts` expects, or null
 /// if dismissed without choosing.
 class MoveCopyConflictSheet extends StatefulWidget {
   final List<MoveCopyConflict> conflicts;