feat(mobile): in-app locale switching

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

apps/mobile/lib/pages/home_page.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../app/locale_controller.dart';
 import '../core/api_client.dart';
 import '../features/uploads/engine_scope.dart';
 import '../l10n/app_localizations.dart';
@@ -56,10 +57,53 @@ class _HomePageState extends State<HomePage> {
   }
 }
 
-/// 设置页（P3 Task 3：修改密码入口 + 退出登录；Task 10 补语言切换持久化）。
+/// 设置页（P3 Task 3：修改密码入口 + 退出登录；Task 10：语言切换）。
 class SettingsTab extends ConsumerWidget {
   const SettingsTab({super.key});
 
+  /// 档位 → 展示名（语言名固定以自身语言书写，不随当前界面语言变化）。
+  static String _choiceLabel(AppLocalizations l10n, AppLocaleChoice choice) =>
+      switch (choice) {
+        AppLocaleChoice.system => l10n.localeFollowSystem,
+        AppLocaleChoice.zh => l10n.localeChinese,
+        AppLocaleChoice.en => l10n.localeEnglish,
+      };
+
+  Future<void> _pickLocale(
+    BuildContext context,
+    WidgetRef ref,
+    AppLocaleChoice current,
+  ) {
+    return showModalBottomSheet<void>(
+      context: context,
+      showDragHandle: true,
+      builder: (sheetContext) {
+        final l10n = AppLocalizations.of(sheetContext);
+        // 打开时刻的快照即可：选中即 pop，无滞留期间的陈旧选中态。
+        return SafeArea(
+          child: RadioGroup<AppLocaleChoice>(
+            groupValue: current,
+            onChanged: (choice) {
+              if (choice == null) return;
+              ref.read(localeSettingsProvider.notifier).setChoice(choice);
+              Navigator.of(sheetContext).pop();
+            },
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                for (final choice in AppLocaleChoice.values)
+                  RadioListTile<AppLocaleChoice>(
+                    value: choice,
+                    title: Text(_choiceLabel(l10n, choice)),
+                  ),
+              ],
+            ),
+          ),
+        );
+      },
+    );
+  }
+
   Future<void> _logout(BuildContext context, WidgetRef ref) async {
     // api.logout 尽力而为（服务端撤销失败也照常本地清会话——客户端内已处理）。
     await ref.read(apiClientProvider).logout();
@@ -72,6 +116,7 @@ class SettingsTab extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final l10n = AppLocalizations.of(context);
+    final choice = ref.watch(localeSettingsProvider);
     return Scaffold(
       appBar: AppBar(title: Text(l10n.settings)),
       body: ListView(
@@ -79,7 +124,8 @@ class SettingsTab extends ConsumerWidget {
           ListTile(
             leading: const Icon(Icons.language),
             title: Text(l10n.language),
-            subtitle: Text(l10n.localeFollowSystem),
+            subtitle: Text(_choiceLabel(l10n, choice)),
+            onTap: () => _pickLocale(context, ref, choice),
           ),
           ListTile(
             leading: const Icon(Icons.password_outlined),