fix: 修复内存泄漏和测试初始化问题

- 修复 AiTagSettingsViewModel 中 Command 未正确释放导致的内存泄漏
- 修复单元测试中缺少 TestWidgetsFlutterBinding.ensureInitialized() 的问题

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: shadowfish07

lib/ui/settings/view_models/ai_tag_settings_viewmodel.dart

@@ -66,6 +66,7 @@ class AiTagSettingsViewModel extends ChangeNotifier {
   @override
   void dispose() {
     _settingsSubscription?.cancel();
+    saveAiTagTargetLanguage.dispose();
     super.dispose();
   }
 }

test/unit/data/service/download_service_test.dart

@@ -4,6 +4,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:readeck_app/data/service/download_service.dart';
 
 void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
   group('DownloadService', () {
     late DownloadService service;
 

test/unit/domain/use_cases/app_update_use_case_test.dart

@@ -5,6 +5,7 @@ import 'package:readeck_app/domain/models/update/update_info.dart';
 import 'package:readeck_app/domain/use_cases/app_update_use_case.dart';
 
 void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
   group('AppUpdateUseCase', () {
     late AppUpdateUseCase useCase;
     late UpdateInfo testUpdateInfo;