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/views/activity_view.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 import 'package:provider/provider.dart';
-import '../providers/server_provider.dart';
+import '../providers/files_controller.dart';
 import '../widgets/more_tabs_button.dart';
 import '../widgets/profile_avatar_button.dart';
 import '../widgets/synced_header_scaffold.dart';
@@ -15,13 +15,13 @@ class ActivityView extends StatelessWidget {
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
     final colorScheme = theme.colorScheme;
-    final provider = context.watch<ServerProvider>();
-    final activities = provider.activities;
+    final files = context.watch<FilesController>();
+    final activities = files.activities;
 
     final List<Widget> contentSlivers = [
       const SliverToBoxAdapter(child: SizedBox(height: 16)),
 
-      if (provider.isLoading && activities.isEmpty)
+      if (files.isLoading && activities.isEmpty)
         const SliverFillRemaining(
           hasScrollBody: false,
           child: Center(child: CircularProgressIndicator()),
@@ -134,7 +134,6 @@ class ActivityView extends StatelessWidget {
 
     return SyncedHeaderScaffold(
       scrollController: scrollController,
-      provider: provider,
       actions: const [MoreTabsButton(), ProfileAvatarButton()],
       contentSlivers: contentSlivers,
     );