refactor: 管理登录页移除默认账号密码预填与文案提示

- 用户名初始值由 admin 改为空
- 删除页脚「默认账号：admin / admin123」文案

client/src/pages/admin/Login.tsx

@@ -6,7 +6,7 @@ import Brand from '../../components/Brand';
 import { authApi, errMsg } from '../../api';
 
 export default function Login() {
-  const [username, setUsername] = useState('admin');
+  const [username, setUsername] = useState('');
   const [password, setPassword] = useState('');
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState('');
@@ -56,8 +56,7 @@ export default function Login() {
         </div>
 
         <div className="mt-6 text-center text-sm text-gray-500">
-          <p>默认账号：admin / admin123</p>
-          <a href="/u" className="inline-block mt-2 underline underline-offset-2 hover:text-[#275354] hover:decoration-[#275354]">← 返回用户端</a>
+          <a href="/u" className="inline-block underline underline-offset-2 hover:text-[#275354] hover:decoration-[#275354]">← 返回用户端</a>
         </div>
       </form>
     </div>