fix(mobile): use external storage for APK download and close app on install

- Change download path from getApplicationDocumentsDirectory to
  getExternalStorageDirectory for proper FileProvider support
- Add finishAffinity() after starting APK install intent

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/android/app/src/main/kotlin/com/rwadurian/rwa_android_app/MainActivity.kt

@@ -81,5 +81,8 @@ class MainActivity : FlutterActivity() {
 
         intent.setDataAndType(apkUri, "application/vnd.android.package-archive")
         startActivity(intent)
+
+        // 关闭当前应用，让系统完成安装
+        finishAffinity()
     }
 }

frontend/mobile-app/lib/core/updater/download_manager.dart

@@ -48,8 +48,8 @@ class DownloadManager {
       _status = DownloadStatus.downloading;
       _cancelToken = CancelToken();
 
-      // 使用应用专属目录（无需额外权限）
-      final dir = await getApplicationDocumentsDirectory();
+      // 使用外部缓存目录（FileProvider 标准支持路径）
+      final dir = await getExternalStorageDirectory() ?? await getApplicationSupportDirectory();
       final savePath = '${dir.path}/app_update.apk';
       final tempPath = '${dir.path}/app_update.apk.tmp';
       final file = File(savePath);
@@ -186,7 +186,7 @@ class DownloadManager {
   /// 删除已下载的 APK 文件和临时文件
   Future<void> cleanupDownloadedApk() async {
     try {
-      final dir = await getApplicationDocumentsDirectory();
+      final dir = await getExternalStorageDirectory() ?? await getApplicationSupportDirectory();
       final file = File('${dir.path}/app_update.apk');
       final tempFile = File('${dir.path}/app_update.apk.tmp');
 
@@ -206,7 +206,7 @@ class DownloadManager {
   /// 清除断点续传的临时文件（用于强制重新下载）
   Future<void> clearPartialDownload() async {
     try {
-      final dir = await getApplicationDocumentsDirectory();
+      final dir = await getExternalStorageDirectory() ?? await getApplicationSupportDirectory();
       final tempFile = File('${dir.path}/app_update.apk.tmp');
       if (await tempFile.exists()) {
         await tempFile.delete();