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

docs/protocols/profile/profile-protocol.md

@@ -46,8 +46,7 @@ Response:
   "settings": {
     "version": 1,
     "preferences": {
-      "interface_language": "zh-CN",
-      "ai_language": "zh-CN",
+      "language": "zh-CN",
       "timezone": "Asia/Shanghai",
       "country": "CN"
     },
@@ -111,8 +110,7 @@ Request:
   "settings": {
     "version": 1,
     "preferences": {
-      "interface_language": "zh-CN",
-      "ai_language": "zh-CN",
+      "language": "zh-CN",
       "timezone": "Asia/Shanghai",
       "country": "CN"
     },