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/frontend/app/(app)/page.tsx

@@ -9,6 +9,7 @@ import { ScrollArea } from '@/components/ui/scroll-area';
 import { Separator } from '@/components/ui/separator';
 import {
   cancelDelegation,
+  createSession,
   deleteSession,
   getSession,
   getStatus,
@@ -204,11 +205,14 @@ export default function ChatPage() {
   }, [loadSessions]);
 
   useEffect(() => {
+    clearMessages();
+    setIsLoading(false);
+    setIsThinking(false);
     resetProcessState();
     const wsSessionId = sessionId.startsWith('web:') ? sessionId.slice(4) : sessionId;
     wsManager.connect(wsSessionId);
     loadSessionMessages(sessionId);
-  }, [loadSessionMessages, resetProcessState, sessionId]);
+  }, [clearMessages, loadSessionMessages, resetProcessState, sessionId, setIsLoading, setIsThinking]);
 
   useEffect(() => {
     const unsubStatus = wsManager.onStatusChange(async (status) => {
@@ -338,6 +342,10 @@ export default function ChatPage() {
         setIsThinking(false);
         setIsLoading(false);
         if (result.response) {
+          if (useChatStore.getState().sessionId !== sessionId) {
+            await loadSessions();
+            return;
+          }
           addMessage({
             role: 'assistant',
             content: result.response,
@@ -351,6 +359,9 @@ export default function ChatPage() {
       } catch {
         setIsThinking(false);
         setIsLoading(false);
+        if (useChatStore.getState().sessionId !== sessionId) {
+          return;
+        }
         addMessage({
           role: 'assistant',
           content: '发送失败，请检查后端服务是否正在运行。',
@@ -413,11 +424,17 @@ export default function ChatPage() {
     }
   }, [sessionId]);
 
-  const handleNewSession = () => {
+  const handleNewSession = async () => {
     const id = `web:${Date.now()}`;
     setSessionId(id);
     clearMessages();
     resetProcessState();
+    try {
+      await createSession(id);
+    } catch {
+      // ignore transient create failures; first message can still create the session server-side
+    }
+    void loadSessions();
   };
 
   const handleDeleteSession = async (key: string, e: React.MouseEvent) => {