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/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>
+  )
 }