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/profile_avatar_button.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
-import '../providers/server_provider.dart';
+import '../providers/session_controller.dart';
 import '../views/account_view.dart';
 
 /// Top-bar avatar button that opens the account screen. Replaces the old
@@ -20,7 +20,7 @@ class ProfileAvatarButton extends StatelessWidget {
     final velocity = details.primaryVelocity ?? 0;
     if (velocity.abs() < 250) return;
 
-    final provider = context.read<ServerProvider>();
+    final provider = context.read<SessionController>();
     final target = velocity < 0
         ? await provider.cycleToNextAccount()
         : await provider.cycleToPreviousAccount();
@@ -37,7 +37,7 @@ class ProfileAvatarButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final provider = context.watch<ServerProvider>();
+    final provider = context.watch<SessionController>();
     final colorScheme = Theme.of(context).colorScheme;
     final initial = provider.username.isNotEmpty
         ? provider.username[0].toUpperCase()