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();