feat: Flutter 客户端 App + Web UI APK 下载入口

- 新增 Flutter 跨平台客户端项目 (clients/flutter/)
  - 29 个 Dart 文件: 服务层/状态管理/5个页面/BLE配网
  - BLE 蓝牙配网: 扫描设备、写入WiFi凭据、配网状态监听
  - HTTP API 客户端: 覆盖全部端点 (播放/场景/WiFi/视频/配置/文件/插件)
  - WebSocket 实时通信: 事件流 + 自动重连
  - 暗色主题 Material 3 UI, 中文界面
  - Android 配置: minSdkVersion 21, BLE/网络权限
  - PRD 产品需求文档 + 开发任务看板
- Web UI 添加 APK 下载入口 (routes.rs)
  - 下载弹窗 + 二维码 + /download/{filename} 静态文件路由
- BLE 插件增加自动重连循环 (ble/mod.rs)
- BLE 默认设备名修正为 'Showen' (config.rs)

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

clients/flutter/lib/main.dart

@@ -0,0 +1,130 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import 'package:provider/provider.dart';
+
+import 'providers/device_provider.dart';
+import 'providers/player_provider.dart';
+import 'providers/wifi_provider.dart';
+import 'screens/app_shell.dart';
+import 'screens/ble_provision_screen.dart';
+import 'screens/home_screen.dart';
+import 'screens/network_screen.dart';
+import 'screens/playback_screen.dart';
+import 'screens/settings_screen.dart';
+import 'screens/trigger_screen.dart';
+import 'services/http_api_service.dart';
+import 'services/web_socket_service.dart';
+import 'theme/app_theme.dart';
+
+void main() {
+  final httpApiService = HttpApiService(baseUrl: 'http://127.0.0.1:8080');
+  final webSocketService = WebSocketService();
+
+  runApp(
+    MultiProvider(
+      providers: [
+        ChangeNotifierProvider<DeviceProvider>(
+          create: (_) => DeviceProvider(
+            httpApiService: httpApiService,
+            webSocketService: webSocketService,
+          )..initialize(),
+        ),
+        ChangeNotifierProvider<PlayerProvider>(
+          create: (_) => PlayerProvider(
+            httpApiService: httpApiService,
+            webSocketService: webSocketService,
+          )
+            ..bootstrap(),
+        ),
+        ChangeNotifierProvider<WifiProvider>(
+          create: (_) => WifiProvider(
+            httpApiService: httpApiService,
+            webSocketService: webSocketService,
+          )
+            ..bootstrap(),
+        ),
+      ],
+      child: const ShowenApp(),
+    ),
+  );
+}
+
+final GoRouter _router = GoRouter(
+  initialLocation: '/',
+  routes: [
+    StatefulShellRoute.indexedStack(
+      builder: (context, state, navigationShell) =>
+          AppShell(navigationShell: navigationShell),
+      branches: [
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/',
+              name: 'home',
+              builder: (context, state) => const HomeScreen(),
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/playback',
+              name: 'playback',
+              builder: (context, state) => const PlaybackScreen(),
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/trigger',
+              name: 'trigger',
+              builder: (context, state) => const TriggerScreen(),
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/network',
+              name: 'network',
+              builder: (context, state) => const NetworkScreen(),
+              routes: [
+                GoRoute(
+                  path: 'ble-provision',
+                  name: 'ble-provision',
+                  builder: (context, state) => const BleProvisionScreen(),
+                ),
+              ],
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/settings',
+              name: 'settings',
+              builder: (context, state) => const SettingsScreen(),
+            ),
+          ],
+        ),
+      ],
+    ),
+  ],
+);
+
+class ShowenApp extends StatelessWidget {
+  const ShowenApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp.router(
+      title: 'ShowenV2',
+      debugShowCheckedModeBanner: false,
+      themeMode: ThemeMode.dark,
+      darkTheme: AppTheme.dark(),
+      theme: AppTheme.dark(),
+      routerConfig: _router,
+    );
+  }
+}