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/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 }),