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>

.claude/context/styling.md

@@ -57,15 +57,15 @@ widgets. Key points:
   new blur/shadow combo.
 - [`FloatingBottomNavBar`](../../lib/widgets/floating_bottom_bar.dart) — the
   main tab bar; opacity/blur are user-adjustable settings
-  (`ServerProvider.bottomBarOpacity`/`bottomBarBlur`), not constants — pull
-  new adjustable visual knobs from the provider the same way rather than
-  hardcoding them.
+  (`SettingsController.bottomBarOpacity`/`bottomBarBlur`), not constants —
+  pull new adjustable visual knobs from `SettingsController` the same way
+  rather than hardcoding them.
 - [`SyncedHeaderScaffold`](../../lib/widgets/synced_header_scaffold.dart) —
   the pull-to-sync `CustomScrollView` header shared by 5 of the 6 tabs (see
   `architecture.md`); also where the pull-to-refresh gesture thresholds and
   the classic Material refresh spinner live. Its persistent chip/panel
   (icon + "Sync off"/"Syncing…"/"Synced"/"Sync issue") reflects device-sync
-  status (`ServerProvider.syncHeaderStatus`), not the WebDAV-refresh
+  status (`SyncStatusController.syncHeaderStatus`), not the WebDAV-refresh
   loading state the pull gesture itself triggers - that has its own,
   separate floating spinner bubble, so nothing was lost by handing the
   persistent text/icon over.