feat(frontend): add React SPA with chat, auth, and admin pages

- Vite 6 + React 19 + TypeScript + Tailwind 4 + Zustand 5
- Login page with JWT auth
- Chat interface: MessageList, ChatInput, ChatContainer
- WebSocket hook with auto-reconnect and exponential backoff
- Sidebar with new chat button + session navigation
- Admin dashboard placeholder
- 57 modules, 242KB JS, TypeScript zero errors

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

@@ -0,0 +1,35 @@
+import { useNavigate } from 'react-router-dom'
+import { useChatStore } from '../../store/chatStore'
+import { api } from '../../api/client'
+
+export default function Sidebar() {
+  const navigate = useNavigate()
+  const { currentSessionId, setSessionId, clearMessages } = useChatStore()
+
+  const handleNewChat = async () => {
+    try {
+      const { session_id } = await api.sessions.create()
+      setSessionId(session_id)
+      clearMessages()
+      navigate(`/chat/${session_id}`)
+    } catch { /* ignore */ }
+  }
+
+  return (
+    <aside className="w-56 bg-gray-900 border-r border-gray-800 flex flex-col shrink-0">
+      <div className="p-3">
+        <button onClick={handleNewChat}
+          className="w-full py-2 bg-purple-600 hover:bg-purple-500 rounded text-sm font-medium transition-colors">
+          + 新对话
+        </button>
+      </div>
+      <div className="flex-1 overflow-auto px-2 text-sm text-gray-400">
+        {currentSessionId ? (
+          <p className="px-2 py-1 truncate text-xs">会话: {currentSessionId.slice(0, 8)}...</p>
+        ) : (
+          <p className="px-2 py-1 text-xs">暂无对话</p>
+        )}
+      </div>
+    </aside>
+  )
+}