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/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),
     );
   }
 }