Add admin route and page

app/(backend)/admin/page.tsx

@@ -0,0 +1,7 @@
+export default function AdminPage() {
+  return (
+    <section className="container">
+      <h1>Admin Page</h1>
+    </section>
+  );
+}

lib/constans.ts

@@ -1,5 +1,6 @@
 export const NAV_ITEMS = [
-  { label: 'Home', href: '/' },
+  { label: "Home", href: "/" },
-  { label: 'About', href: '/about' },
+  { label: "About", href: "/about" },
-  { label: 'Contact', href: '/contact' },
+  { label: "Contact", href: "/contact" },
-]
+  { label: "Admin", href: "/admin" },
+];