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/Header.tsx

@@ -0,0 +1,14 @@
+import { Link, useLocation } from 'react-router-dom'
+
+export default function Header({ username, onLogout }: { username: string; onLogout: () => void }) {
+  return (
+    <header className="h-12 bg-gray-900 border-b border-gray-800 flex items-center px-4 justify-between shrink-0">
+      <Link to="/chat" className="font-bold text-purple-400 text-lg">Cyrene</Link>
+      <div className="flex items-center gap-3 text-sm text-gray-400">
+        <Link to="/admin/dashboard" className="hover:text-gray-200">管理</Link>
+        <span>{username}</span>
+        <button onClick={onLogout} className="hover:text-red-400">退出</button>
+      </div>
+    </header>
+  )
+}