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/models/video_item.dart

@@ -0,0 +1,23 @@
+class VideoItem {
+  const VideoItem({required this.name, required this.size});
+
+  final String name;
+  final int size;
+
+  factory VideoItem.fromJson(Map<String, dynamic> json) {
+    return VideoItem(
+      name: json['name'] as String? ?? '',
+      size: json['size'] as int? ?? 0,
+    );
+  }
+
+  String get sizeLabel {
+    if (size >= 1024 * 1024) {
+      return '${(size / (1024 * 1024)).toStringAsFixed(1)} MB';
+    }
+    if (size >= 1024) {
+      return '${(size / 1024).toStringAsFixed(1)} KB';
+    }
+    return '$size B';
+  }
+}