refactor(settings): 统一语言设置，合并 interface_language 和 ai_language

- 后端 Schema 将 interface_language 和 ai_language 合并为 language
- 前端设置界面只保留一个语言选项
- AI 回复语言统一使用 language 设置
- 更新协议文档
- 新增数据库迁移脚本

apps/lib/features/settings/data/apis/profile_api.dart

@@ -41,10 +41,8 @@ class ProfileApi {
       'settings': {
         'version': settings.version,
         'preferences': {
-          'interface_language': settings.preferences.interfaceLanguage,
-          'ai_language': settings.preferences.aiLanguage,
+          'language': settings.preferences.language,
           'timezone': settings.preferences.timezone,
-          'country': settings.preferences.country,
         },
         'privacy': {
           'can_sell': settings.privacy.canSell,
@@ -109,12 +107,9 @@ class ProfileApi {
         : null;
     final preferences = preferencesRaw is Map<String, dynamic>
         ? PreferenceSettings(
-            interfaceLanguage:
-                (preferencesRaw['interface_language'] as String?) ?? 'zh-CN',
-            aiLanguage: (preferencesRaw['ai_language'] as String?) ?? 'zh-CN',
+            language: (preferencesRaw['language'] as String?) ?? 'zh-CN',
             timezone:
                 (preferencesRaw['timezone'] as String?) ?? 'Asia/Shanghai',
-            country: (preferencesRaw['country'] as String?) ?? 'US',
           )
         : const PreferenceSettings();
 

apps/lib/features/settings/data/models/profile_settings.dart

@@ -13,28 +13,20 @@ String displayLanguageLabel(AppLocalizations l10n, String languageTag) {
 
 class PreferenceSettings {
   const PreferenceSettings({
-    this.interfaceLanguage = 'zh-CN',
-    this.aiLanguage = 'zh-CN',
+    this.language = 'zh-CN',
     this.timezone = 'Asia/Shanghai',
-    this.country = 'US',
   });
 
-  final String interfaceLanguage;
-  final String aiLanguage;
+  final String language;
   final String timezone;
-  final String country;
 
   PreferenceSettings copyWith({
-    String? interfaceLanguage,
-    String? aiLanguage,
+    String? language,
     String? timezone,
-    String? country,
   }) {
     return PreferenceSettings(
-      interfaceLanguage: interfaceLanguage ?? this.interfaceLanguage,
-      aiLanguage: aiLanguage ?? this.aiLanguage,
+      language: language ?? this.language,
       timezone: timezone ?? this.timezone,
-      country: country ?? this.country,
     );
   }
 }
@@ -151,7 +143,7 @@ class ProfileSettingsV1 {
   factory ProfileSettingsV1.defaultsForLocale(Locale locale) {
     final tag = languageTagFromLocale(locale);
     return ProfileSettingsV1(
-      preferences: PreferenceSettings(interfaceLanguage: tag, aiLanguage: tag),
+      preferences: PreferenceSettings(language: tag),
     );
   }
 }

apps/lib/features/settings/presentation/screens/general_settings_screen.dart

@@ -54,40 +54,20 @@ class _GeneralSettingsScreenState extends State<GeneralSettingsScreen> {
             children: [
               SettingsMenuTile(
                 icon: Icons.language_rounded,
-                title: l10n.settingsInterfaceLanguage,
+                title: l10n.settingsLanguage,
                 subtitle: displayLanguageLabel(
                   l10n,
-                  _settings.preferences.interfaceLanguage,
-                ),
-                tint: colors.primary,
-                background: colors.surfaceContainerHighest,
-                onTap: () => _selectLanguage(
-                  _settings.preferences.interfaceLanguage,
-                  (lang) => setState(() {
-                    _settings = _settings.copyWith(
-                      preferences: _settings.preferences.copyWith(
-                        interfaceLanguage: lang,
-                      ),
-                    );
-                  }),
-                ),
-              ),
-              SettingsMenuTile(
-                icon: Icons.smart_toy_rounded,
-                title: l10n.settingsAiLanguage,
-                subtitle: displayLanguageLabel(
-                  l10n,
-                  _settings.preferences.aiLanguage,
+                  _settings.preferences.language,
                 ),
                 tint: colors.primary,
                 background: colors.surfaceContainerHighest,
                 showDivider: false,
                 onTap: () => _selectLanguage(
-                  _settings.preferences.aiLanguage,
+                  _settings.preferences.language,
                   (lang) => setState(() {
                     _settings = _settings.copyWith(
                       preferences: _settings.preferences.copyWith(
-                        aiLanguage: lang,
+                        language: lang,
                       ),
                     );
                   }),