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/providers/recent_controller.dart

@@ -0,0 +1,59 @@
+import 'dart:async';
+import 'package:flutter/material.dart';
+import '../models/nextcloud_item.dart';
+import 'session_controller.dart';
+
+/// The Recent Files tab. Kept separate from Files' own loading/error
+/// state, likewise.
+class RecentController extends ChangeNotifier {
+  final SessionController session;
+
+  List<NextcloudItem> _items = [];
+  bool _isLoading = false;
+  String? _errorMessage;
+
+  RecentController(this.session) {
+    session.addAccountClearedListener(_onAccountCleared);
+    session.addAccountActivatedListener(_onAccountActivated);
+  }
+
+  List<NextcloudItem> get items => _items;
+  bool get isLoading => _isLoading;
+  String? get errorMessage => _errorMessage;
+
+  void _onAccountCleared() {
+    _items = [];
+    _isLoading = false;
+    _errorMessage = null;
+    notifyListeners();
+  }
+
+  void _onAccountActivated() {
+    unawaited(fetchAll());
+  }
+
+  Future<void> fetchAll() async {
+    final service = session.service;
+    if (!session.isLoggedIn || service == null) return;
+    final gen = session.sessionGeneration;
+
+    _isLoading = true;
+    _errorMessage = null;
+    notifyListeners();
+
+    try {
+      final recent = await service.fetchRecentFiles();
+      if (gen != session.sessionGeneration) return;
+      _items = recent;
+    } catch (e) {
+      if (gen != session.sessionGeneration) return;
+      debugPrint('[RecentController] Error fetching recent files: $e');
+      _errorMessage = e.toString().replaceAll('Exception: ', '');
+    } finally {
+      if (gen == session.sessionGeneration) {
+        _isLoading = false;
+        notifyListeners();
+      }
+    }
+  }
+}