feat(mobile): in-app locale switching

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

apps/mobile/lib/main.dart

@@ -2,7 +2,21 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'app/app.dart';
+import 'app/locale_controller.dart';
 
-void main() {
-  runApp(const ProviderScope(child: PhotoLiveApp()));
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  // 启动先读语言偏好再 runApp：首帧即正确语言（无「system 闪变」，
+  // P3 Task 10）。读取失败由 readPersisted 内部兜底为 system。
+  final localeChoice = await LocaleSettings.readPersisted();
+  runApp(
+    ProviderScope(
+      overrides: [
+        localeSettingsProvider.overrideWith(
+          () => LocaleSettings(initial: localeChoice),
+        ),
+      ],
+      child: const PhotoLiveApp(),
+    ),
+  );
 }