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/widgets/wifi_list_tile.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+
+import '../models/wifi_network.dart';
+import '../theme/app_colors.dart';
+
+class WifiListTile extends StatelessWidget {
+  const WifiListTile({
+    required this.network,
+    required this.onTap,
+    super.key,
+  });
+
+  final WifiNetwork network;
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return Card(
+      child: ListTile(
+        onTap: onTap,
+        contentPadding: const EdgeInsets.symmetric(
+          horizontal: AppSpacing.md,
+          vertical: AppSpacing.xs,
+        ),
+        leading: Container(
+          width: 48,
+          height: 48,
+          decoration: BoxDecoration(
+            color: AppColors.info.withOpacity(0.14),
+            borderRadius: BorderRadius.circular(AppRadius.medium),
+          ),
+          child: const Icon(Icons.wifi_rounded, color: AppColors.info),
+        ),
+        title: Text(network.ssid),
+        subtitle: Text('${network.security} · ${network.signalLabel}'),
+        trailing: const Icon(Icons.chevron_right_rounded),
+      ),
+    );
+  }
+}