feat(ui): 添加明暗主题切换支持

- 创建 ThemeProvider 管理主题状态
- 配置浅色和深色主题（Vercel/Linear 风格）
- 集成 Google Fonts（Inter + JetBrains Mono）
- 在我的页面添加主题切换开关
- 更新颜色系统符合 modernization-v2.md 规范
- 优化间距和圆角系统

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flutter_monisuo/lib/main.dart

@@ -16,6 +16,7 @@ import 'data/services/fund_service.dart';
 import 'providers/auth_provider.dart';
 import 'providers/market_provider.dart';
 import 'providers/asset_provider.dart';
+import 'providers/theme_provider.dart';
 import 'ui/pages/auth/login_page.dart';
 import 'ui/pages/main/main_page.dart';
 
@@ -36,12 +37,17 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MultiProvider(
       providers: _buildProviders(),
-      child: AuthNavigator(
-        child: ShadApp.custom(
-          themeMode: ThemeMode.dark,
-          darkTheme: createAppShadTheme(),
-          appBuilder: _buildMaterialApp,
-        ),
+      child: Consumer<ThemeProvider>(
+        builder: (context, themeProvider, _) {
+          return AuthNavigator(
+            child: ShadApp.custom(
+              themeMode: themeProvider.themeMode,
+              theme: createLightShadTheme(),
+              darkTheme: createDarkShadTheme(),
+              appBuilder: _buildMaterialApp,
+            ),
+          );
+        },
       ),
     );
   }
@@ -50,6 +56,10 @@ class MyApp extends StatelessWidget {
     final dioClient = DioClient();
 
     return [
+      // Theme Provider (必须放在最前面)
+      ChangeNotifierProvider<ThemeProvider>(
+        create: (_) => ThemeProvider()..init(),
+      ),
       // Services
       Provider<DioClient>.value(value: dioClient),
       Provider<UserService>(create: (_) => UserService(dioClient)),