feat(nanobot-web): 添加会话创建接口并优化前端会话管理

- 新增 POST /api/sessions/{key} 接口用于立即创建或持久化会话
- 提取 _serialize_session_detail 函数以统一会话数据序列化逻辑
- 前端添加 createSession API 调用函数
- 实现本地存储中的会话ID持久化功能
- 优化 ChatPage 组件中的会话切换逻辑，确保状态正确重置
- 在消息处理中添加会话ID验证，避免跨会话消息混乱
- 新建会话时主动调用创建API并刷新会话列表

app-instance/backend/nanobot/web/server.py

@@ -1580,11 +1580,8 @@ def _register_routes(app: FastAPI) -> None:
         sm: SessionManager = app.state.session_manager
         return sm.list_sessions()
 
-    @app.get("/api/sessions/{key:path}")
-    async def get_session(key: str):
-        """Get a session's message history."""
-        sm: SessionManager = app.state.session_manager
-        session = sm.get_or_create(key)
+    def _serialize_session_detail(session: Session) -> dict[str, Any]:
+        """Build the filtered session payload returned to the web UI."""
         # Filter out tool messages and assistant messages with tool_calls
         # (intermediate steps), only keep user messages and final assistant replies
         visible_messages = []
@@ -1616,6 +1613,21 @@ def _register_routes(app: FastAPI) -> None:
             "updated_at": session.updated_at.isoformat(),
         }
 
+    @app.post("/api/sessions/{key:path}")
+    async def create_session(key: str):
+        """Create or persist a session immediately."""
+        sm: SessionManager = app.state.session_manager
+        session = sm.get_or_create(key)
+        sm.save(session)
+        return _serialize_session_detail(session)
+
+    @app.get("/api/sessions/{key:path}")
+    async def get_session(key: str):
+        """Get a session's message history."""
+        sm: SessionManager = app.state.session_manager
+        session = sm.get_or_create(key)
+        return _serialize_session_detail(session)
+
     @app.delete("/api/sessions/{key:path}")
     async def delete_session(key: str):
         """Delete a session."""