feat(frontend): session list sidebar and persisted history loading

- Sidebar lists sessions with title/relative time, click to switch,
  hover-to-delete; refreshes after each round so new titles appear
- ChatPage loads message history from GET /sessions/:id/messages when
  entering a session
- WS connect now passes user_id so persisted messages have an owner

frontend/web/src/api/client.ts

@@ -33,9 +33,13 @@ export const api = {
   health: () => request<{ status: string }>('/health'),
 
   sessions: {
-    list: () => request<{ sessions: { id: string; title: string }[] }>('/sessions'),
+    list: () => request<{ sessions: { id: string; title: string; updated_at: string }[] }>('/sessions'),
     create: () => request<{ session_id: string }>('/sessions', { method: 'POST' }),
     delete: (id: string) => request<void>(`/sessions/${id}`, { method: 'DELETE' }),
+    messages: (id: string) =>
+      request<{ messages: { id: number; session_id: string; role: string; content: string; created_at: string }[] }>(`/sessions/${id}/messages`),
+    exportUrl: (id: string, format: 'markdown' | 'txt' | 'json' = 'markdown') =>
+      `${BASE}/sessions/${id}/export?format=${format}`,
   },
 
   admin: {

frontend/web/src/components/layout/Sidebar.tsx

@@ -1,10 +1,35 @@
+import { useCallback, useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
 import { useChatStore } from '../../store/chatStore'
 import { api } from '../../api/client'
 
+interface SessionItem { id: string; title: string; updated_at: string }
+
+function relativeTime(iso: string): string {
+  const diff = Date.now() - new Date(iso).getTime()
+  const min = Math.floor(diff / 60000)
+  if (min < 1) return '刚刚'
+  if (min < 60) return `${min} 分钟前`
+  const hour = Math.floor(min / 60)
+  if (hour < 24) return `${hour} 小时前`
+  return `${Math.floor(hour / 24)} 天前`
+}
+
 export default function Sidebar() {
   const navigate = useNavigate()
-  const { currentSessionId, setSessionId, clearMessages } = useChatStore()
+  const { currentSessionId, setSessionId, clearMessages, messages } = useChatStore()
+  const [sessions, setSessions] = useState<SessionItem[]>([])
+
+  const fetchSessions = useCallback(async () => {
+    try {
+      const data = await api.sessions.list()
+      setSessions(data.sessions || [])
+    } catch { /* ignore */ }
+  }, [])
+
+  useEffect(() => { fetchSessions() }, [fetchSessions])
+  // Refresh the list when a round finishes so new titles/ordering appear
+  useEffect(() => { fetchSessions() }, [messages.length, fetchSessions])
 
   const handleNewChat = async () => {
     try {
@@ -12,6 +37,19 @@ export default function Sidebar() {
       setSessionId(session_id)
       clearMessages()
       navigate(`/chat/${session_id}`)
+      fetchSessions()
+    } catch { /* ignore */ }
+  }
+
+  const handleDelete = async (id: string) => {
+    if (!window.confirm('删除这个会话及其全部消息？')) return
+    try {
+      await api.sessions.delete(id)
+      if (id === currentSessionId) {
+        clearMessages()
+        navigate('/chat')
+      }
+      fetchSessions()
     } catch { /* ignore */ }
   }
 
@@ -25,13 +63,32 @@ export default function Sidebar() {
         </button>
       </div>
 
-      <div className="flex-1 overflow-auto px-2 pb-2">
-        {currentSessionId ? (
-          <div className="px-3 py-2 bg-[var(--mx-bg)] rounded-[14px]">
-            <p className="text-xs text-[var(--mx-text-dim)] truncate">{currentSessionId.slice(0, 12)}...</p>
-          </div>
-        ) : (
+      <div className="flex-1 overflow-auto px-2 pb-2 space-y-1">
+        {sessions.length === 0 ? (
           <p className="px-3 py-2 text-xs text-[var(--mx-text-dim)] text-center">暂无对话</p>
+        ) : (
+          sessions.map((s) => (
+            <div key={s.id}
+              onClick={() => navigate(`/chat/${s.id}`)}
+              className="group px-3 py-2 rounded-[14px] cursor-pointer transition-colors"
+              style={{
+                background: s.id === currentSessionId ? 'color-mix(in srgb, var(--mx-primary) 12%, transparent)' : 'transparent',
+              }}
+            >
+              <div className="flex items-center justify-between gap-1">
+                <p className="text-sm truncate flex-1" style={{ color: 'var(--mx-text)' }}>
+                  {s.title || '未命名会话'}
+                </p>
+                <button
+                  onClick={(e) => { e.stopPropagation(); handleDelete(s.id) }}
+                  className="opacity-0 group-hover:opacity-100 text-xs px-1 transition-opacity"
+                  style={{ color: 'var(--mx-error)' }}
+                  title="删除会话"
+                >✕</button>
+              </div>
+              <p className="text-xs mt-0.5" style={{ color: 'var(--mx-text-dim)' }}>{relativeTime(s.updated_at)}</p>
+            </div>
+          ))
         )}
       </div>
     </aside>

frontend/web/src/hooks/useWebSocket.ts

@@ -1,5 +1,6 @@
 import { useEffect, useRef, useCallback } from 'react'
 import { useChatStore } from '../store/chatStore'
+import { useAuthStore } from '../store/authStore'
 import { getToken } from '../api/client'
 import type { WSMessage } from '../types/chat'
 
@@ -12,7 +13,8 @@ export function useWebSocket() {
     const token = getToken()
     if (!token) return
 
-    const ws = new WebSocket(`ws://localhost:8080/ws/chat?token=${encodeURIComponent(token)}`)
+    const uid = useAuthStore.getState().username || 'web'
+    const ws = new WebSocket(`ws://localhost:8080/ws/chat?token=${encodeURIComponent(token)}&user_id=${encodeURIComponent(uid)}`)
     wsRef.current = ws
 
     ws.onopen = () => {

frontend/web/src/pages/chat/ChatPage.tsx

@@ -2,15 +2,40 @@ import { useEffect } from 'react'
 import { useParams } from 'react-router-dom'
 import { useChatStore } from '../../store/chatStore'
 import ChatContainer from '../../components/chat/ChatContainer'
+import Sidebar from '../../components/layout/Sidebar'
 import { api } from '../../api/client'
+import type { Message } from '../../types/chat'
 
 export default function ChatPage() {
   const { sessionId } = useParams()
-  const { setSessionId } = useChatStore()
+  const { setSessionId, setMessages } = useChatStore()
 
   useEffect(() => {
-    if (sessionId) setSessionId(sessionId)
-  }, [sessionId, setSessionId])
+    if (!sessionId) return
+    setSessionId(sessionId)
 
-  return <ChatContainer />
+    // Load persisted history for the session
+    let cancelled = false
+    api.sessions.messages(sessionId).then((data) => {
+      if (cancelled) return
+      const msgs: Message[] = (data.messages || []).map((m) => ({
+        id: `db-${m.id}`,
+        session_id: m.session_id,
+        role: m.role === 'user' ? 'user' : 'assistant',
+        content: m.content,
+        created_at: m.created_at,
+      }))
+      setMessages(msgs)
+    }).catch(() => { /* history unavailable */ })
+    return () => { cancelled = true }
+  }, [sessionId, setSessionId, setMessages])
+
+  return (
+    <div className="flex h-full">
+      <Sidebar />
+      <div className="flex-1 min-w-0">
+        <ChatContainer />
+      </div>
+    </div>
+  )
 }

frontend/web/src/store/chatStore.ts

@@ -10,6 +10,7 @@ interface ChatState {
 
   setInput: (text: string) => void
   setSessionId: (id: string) => void
+  setMessages: (msgs: Message[]) => void
   addMessage: (msg: Message) => void
   appendToLast: (content: string) => void
   setStreaming: (v: boolean) => void
@@ -26,6 +27,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
 
   setInput: (text) => set({ inputText: text }),
   setSessionId: (id) => set({ currentSessionId: id }),
+  setMessages: (msgs) => set({ messages: msgs }),
   setStreaming: (v) => set({ isStreaming: v }),
   setIotDevices: (d) => set({ iotDevices: d }),