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/android/app/build.gradle

@@ -0,0 +1,37 @@
+plugins {
+    id 'com.android.application'
+    id 'org.jetbrains.kotlin.android'
+    id 'dev.flutter.flutter-gradle-plugin'
+}
+
+android {
+    namespace 'com.showen.flutter'
+    compileSdk flutter.compileSdkVersion
+
+    defaultConfig {
+        applicationId 'com.showen.flutter'
+        minSdkVersion 21
+        targetSdkVersion flutter.targetSdkVersion
+        versionCode flutter.versionCode
+        versionName flutter.versionName
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_17
+        targetCompatibility JavaVersion.VERSION_17
+    }
+
+    kotlinOptions {
+        jvmTarget = '17'
+    }
+
+    buildTypes {
+        release {
+            signingConfig signingConfigs.debug
+        }
+    }
+}
+
+flutter {
+    source '../..'
+}