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

backend/src/core/agentscope/runtime/runner.py

@@ -265,15 +265,15 @@ class AgentScopeRunner:
             emit_text_events=True,
             emit_tool_events=False,
         )
-        ai_language = "zh-CN"
+        language = "zh-CN"
         if user_context.settings is not None:
             prefs = getattr(user_context.settings, "preferences", None)
             if prefs is not None:
-                ai_language = getattr(prefs, "ai_language", "zh-CN") or "zh-CN"
+                language = getattr(prefs, "language", "zh-CN") or "zh-CN"
 
         system_prompt = build_system_prompt(
             agent_type=stage_config.agent_type,
-            ai_language=ai_language,
+            language=language,
             llm_config=stage_config.llm_config,
             tools=None,
             now_utc=datetime.now(timezone.utc),