frontend: remove admin user management page and nav link

- Delete admin user management page
- Remove 管理后台 link from main layout header

frontend/app/(main)/layout.tsx

@@ -22,11 +22,6 @@ export default async function MainLayout({
             <Link href="/dashboard" className="text-sm hover:underline">
               仪表盘
             </Link>
-            {user?.role === "admin" && (
-              <Link href="/admin" className="text-sm hover:underline">
-                管理后台
-              </Link>
-            )}
             <form
               action={async () => {
                 "use server";