feat：简单添加管理员后台功能

client/src/app/AdminLayout.tsx

@@ -0,0 +1,52 @@
+import { Layout, Menu, Typography } from 'antd';
+import { Outlet, useLocation, useNavigate } from 'react-router-dom';
+import { clearAuth } from '../lib/adminAuth';
+
+const { Header, Content, Sider } = Layout;
+
+export function AdminLayout() {
+  const navigate = useNavigate();
+  const location = useLocation();
+
+  const selectedKey = location.pathname.startsWith('/users') ? 'users' : 'users';
+
+  return (
+    <Layout style={{ minHeight: '100vh' }}>
+      <Sider width={220}>
+        <div style={{ padding: 16 }}>
+          <Typography.Title level={5} style={{ color: 'white', margin: 0 }}>
+            Whale Town Admin
+          </Typography.Title>
+        </div>
+        <Menu
+          theme="dark"
+          mode="inline"
+          selectedKeys={[selectedKey]}
+          items={[
+            {
+              key: 'users',
+              label: '用户管理',
+              onClick: () => navigate('/users'),
+            },
+            {
+              key: 'logout',
+              label: '退出登录',
+              onClick: () => {
+                clearAuth();
+                navigate('/login');
+              },
+            },
+          ]}
+        />
+      </Sider>
+      <Layout>
+        <Header style={{ background: 'white', display: 'flex', alignItems: 'center' }}>
+          <Typography.Text>后台管理</Typography.Text>
+        </Header>
+        <Content style={{ padding: 16 }}>
+          <Outlet />
+        </Content>
+      </Layout>
+    </Layout>
+  );
+}