fix(android): add app_flutter path for FileProvider

APK downloaded to getApplicationDocumentsDirectory() which maps to
files/app_flutter/ on Android. FileProvider needs this path configured
to grant URI permission for APK installation.

🤖 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/res/xml/file_paths.xml

@@ -2,6 +2,8 @@
 <paths>
     <!-- 应用内部文件目录 -->
     <files-path name="internal_files" path="." />
+    <!-- Flutter Documents 目录 (getApplicationDocumentsDirectory) -->
+    <files-path name="app_flutter" path="app_flutter/" />
     <!-- 应用缓存目录 -->
     <cache-path name="cache" path="." />
     <!-- 应用外部文件目录 -->