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/lib/api.ts

@@ -509,6 +509,10 @@ export async function listSessions(): Promise<Session[]> {
   return fetchJSON('/api/sessions');
 }
 
+export async function createSession(key: string): Promise<SessionDetail> {
+  return fetchJSON(`/api/sessions/${encodeURIComponent(key)}`, { method: 'POST' });
+}
+
 export async function getSession(key: string): Promise<SessionDetail> {
   return fetchJSON(`/api/sessions/${encodeURIComponent(key)}`);
 }

app-instance/frontend/lib/store.ts

@@ -13,6 +13,23 @@ import type {
 } from '@/types';
 import type { WsStatus } from '@/lib/api';
 
+const ACTIVE_SESSION_STORAGE_KEY = 'nanobot_active_session_id';
+
+function getInitialSessionId(): string {
+  if (typeof window === 'undefined') {
+    return 'web:default';
+  }
+  const saved = window.localStorage.getItem(ACTIVE_SESSION_STORAGE_KEY)?.trim();
+  return saved || 'web:default';
+}
+
+function persistSessionId(id: string): void {
+  if (typeof window === 'undefined') {
+    return;
+  }
+  window.localStorage.setItem(ACTIVE_SESSION_STORAGE_KEY, id);
+}
+
 interface ChatStore {
   user: AuthUser | null;
   isAuthLoading: boolean;
@@ -105,7 +122,7 @@ function createEventId(event: ProcessWsEvent): string {
 export const useChatStore = create<ChatStore>((set) => ({
   user: null,
   isAuthLoading: true,
-  sessionId: 'web:default',
+  sessionId: getInitialSessionId(),
   messages: [],
   isLoading: false,
   streamingContent: '',
@@ -125,7 +142,10 @@ export const useChatStore = create<ChatStore>((set) => ({
 
   setUser: (user) => set({ user }),
   setIsAuthLoading: (loading) => set({ isAuthLoading: loading }),
-  setSessionId: (id) => set({ sessionId: id }),
+  setSessionId: (id) => {
+    persistSessionId(id);
+    set({ sessionId: id });
+  },
   setMessages: (msgs) => set({ messages: msgs }),
   addMessage: (msg) => set((s) => ({ messages: [...s.messages, msg] })),
   setIsLoading: (loading) => set({ isLoading: loading }),