feat(mobile): in-app locale switching

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

apps/mobile/test/helpers/fakes.dart

@@ -10,6 +10,7 @@ import 'dart:convert';
 
 import 'package:dio/dio.dart';
 import 'package:drift/drift.dart' show driftRuntimeOptions;
+import 'package:photo_live/app/locale_controller.dart';
 import 'package:photo_live/core/api_client.dart';
 import 'package:photo_live/core/session_store.dart';
 import 'package:photo_live/features/uploads/providers.dart';
@@ -29,6 +30,24 @@ class FakeSessionStore implements SessionStore {
   Future<void> clear() async => session = null;
 }
 
+/// 内存语言偏好存储（测试替身，P3 Task 10）。
+class FakeLocaleStore implements LocalePrefsStore {
+  /// 当前持久值（read 返回）；null = 未设置（跟随系统）。
+  String? value;
+
+  /// 按序记录全部写入（断言持久化调用用）。
+  final List<String> writes = <String>[];
+
+  @override
+  Future<String?> read() async => value;
+
+  @override
+  Future<void> write(String value) async {
+    writes.add(value);
+    this.value = value;
+  }
+}
+
 /// Widget 测试通用 overrides（P3 Task 9 起，登录页/主壳会触达
 /// sessionAccountProvider 与任务库 Provider——真实 flutter_secure_storage
 /// 与 path_provider 的平台通道在测试环境挂起，凡 pump 到相关页面均需