add not found page

app/not-found.tsx

@@ -0,0 +1,15 @@
+import Link from 'next/link'
+
+export default function NotFound() {
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center">
+      <h2 className="text-2xl font-bold mb-4">404 - Page Not Found</h2>
+      <p className="text-gray-600 mb-4">
+        The page you're looking for doesn't exist.
+      </p>
+      <Link href="/" className="text-blue-600 hover:text-blue-800 underline">
+        Return Home
+      </Link>
+    </div>
+  )
+}