feat(flutter): 添加TaskProvider到主应用

- 在MultiProvider中注册TaskProvider
- 用于管理异步任务状态
- 支持文件解析、OCR、NER等任务的状态跟踪

frontend_flutter/lib/main.dart

@@ -5,6 +5,7 @@ import 'router/app_router.dart';
 import 'providers/document_provider.dart';
 import 'providers/element_provider.dart';
 import 'providers/annotation_provider.dart';
+import 'providers/task_provider.dart';
 
 void main() {
   runApp(const MyApp());
@@ -20,6 +21,7 @@ class MyApp extends StatelessWidget {
         ChangeNotifierProvider(create: (_) => DocumentProvider()),
         ChangeNotifierProvider(create: (_) => ElementProvider()),
         ChangeNotifierProvider(create: (_) => AnnotationProvider()),
+        ChangeNotifierProvider(create: (_) => TaskProvider()),
       ],
       child: MaterialApp.router(
         title: '灵越智报',