feat(mobile): in-app locale switching

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

apps/mobile/test/pages/settings_tab_test.dart

@@ -8,6 +8,7 @@ import 'package:dio/dio.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
+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/l10n/app_localizations.dart';
@@ -35,13 +36,18 @@ void main() {
           ..httpClientAdapter = adapter,
       );
 
-  Future<void> pumpHome(WidgetTester tester, ApiClient client) async {
+  Future<void> pumpHome(
+    WidgetTester tester,
+    ApiClient client, {
+    List<dynamic> extraOverrides = const <dynamic>[],
+  }) async {
     await tester.pumpWidget(
       ProviderScope(
         // Task 9 起主壳含队列页（触达 sessionAccountProvider/TaskStore），
         // 真实平台通道在测试环境挂起，必须注入替身。
         overrides: [
           ...pageTestOverrides(session: store, client: client),
+          ...extraOverrides,
         ],
         child: MaterialApp(
           localizationsDelegates: AppLocalizations.localizationsDelegates,
@@ -92,4 +98,39 @@ void main() {
     expect(find.byType(LoginPage), findsOneWidget);
     expect(find.byType(HomePage), findsNothing, reason: '主壳已出栈');
   });
+
+  testWidgets('语言切换：底部弹层三选一 → 更新档位 + 持久化（P3 Task 10）',
+      (tester) async {
+    final adapter = FakeAdapter((_) => throw StateError('不应发起请求'));
+    final localeStore = FakeLocaleStore();
+    await pumpHome(
+      tester,
+      clientOf(adapter),
+      extraOverrides: [
+        localeSettingsProvider.overrideWith(
+          () => LocaleSettings(store: localeStore),
+        ),
+      ],
+    );
+
+    // 本 harness 的 MaterialApp 固定 en（live 全 App 重建在
+    // app/locale_test.dart 的 PhotoLiveApp 用例覆盖）。
+    expect(find.text('Language'), findsOneWidget);
+    expect(find.text('Follow system'), findsOneWidget, reason: '默认档位副标题');
+
+    await tester.tap(find.text('Language'));
+    await tester.pumpAndSettle();
+
+    // 弹层三选项（语言名以自身语言书写，跨 locale 不变）。
+    // 「Follow system」出现两次：主档 tile 副标题 + 弹层选项。
+    expect(find.text('Follow system'), findsNWidgets(2));
+    expect(find.text('中文'), findsOneWidget);
+    expect(find.text('English'), findsOneWidget);
+
+    await tester.tap(find.text('中文'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('中文'), findsOneWidget, reason: '副标题更新为当前档位');
+    expect(localeStore.writes, ['zh'], reason: '选择写入 app.locale');
+  });
 }