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/move_copy_result.dart

@@ -2,7 +2,7 @@ import 'nextcloud_item.dart';
 
 /// A resolution choice for one conflicting item in a Move/Copy batch -
 /// keyed by `item.id` in [ConflictChoice] maps passed to
-/// `ServerProvider.resolveConflicts`.
+/// `ItemOperations.resolveConflicts`.
 enum ConflictChoice { overwrite, keepBoth, skip }
 
 /// One item that couldn't be moved/copied because something with the same
@@ -14,7 +14,7 @@ class MoveCopyConflict {
 }
 
 /// Outcome of a Move/Copy batch (an initial attempt via
-/// `ServerProvider.moveItems`/`copyItems`, or a follow-up
+/// `ItemOperations.moveItems`/`copyItems`, or a follow-up
 /// `resolveConflicts` call). [blockedReason] is set instead of attempting
 /// anything at all when the destination itself is invalid (e.g. moving a
 /// folder into its own subfolder).