feat: Flutter APK 编译成功 + Gradle 配置修复 + APK 下载部署 + 待优化清单

- 通过 qemu-user-static 实现 ARM64 主机编译 Android APK (51MB)
- 修复 Gradle: Aliyun 镜像 + PREFER_SETTINGS + JVM 内存 1536M
- 部署 APK 到 configs/downloads/, Web 下载接口已验证 (HTTP 200)
- 新增 Flutter TODO.md: 10项待优化 (P0/P1/P2 分级)
- 新增 pm_soul.md, 更新 routes.rs APK 下载路由

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

clients/flutter/android/app/build.gradle

@@ -10,7 +10,7 @@ android {
 
     defaultConfig {
         applicationId 'com.showen.flutter'
-        minSdkVersion 21
+        minSdkVersion flutter.minSdkVersion
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutter.versionCode
         versionName flutter.versionName