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/pages/admin/DashboardPage.tsx

@@ -0,0 +1,17 @@
+export default function DashboardPage() {
+  return (
+    <div className="p-6 max-w-3xl mx-auto">
+      <h1 className="text-xl font-bold mb-4">管理面板</h1>
+      <div className="grid grid-cols-2 gap-4">
+        <div className="bg-gray-800 rounded-lg p-4">
+          <p className="text-gray-400 text-sm">服务状态</p>
+          <p className="text-green-400 text-lg font-bold mt-1">运行中</p>
+        </div>
+        <div className="bg-gray-800 rounded-lg p-4">
+          <p className="text-gray-400 text-sm">版本</p>
+          <p className="text-purple-400 text-lg font-bold mt-1">2.0.0-dev</p>
+        </div>
+      </div>
+    </div>
+  )
+}