add custom 404 page

src/app/not-found.tsx

@@ -0,0 +1,15 @@
+import Link from "next/link";
+
+export default async function NotFound() {
+    return (
+        <div className="w-full max-w-md mx-auto p-3 md:mt-32 text-center">
+            <h1 className="text-6xl font-bold text-gray-800 mb-4">404</h1>
+            <h2 className="text-2xl font-semibold text-gray-600 mb-4">
+                Page Not Found
+            </h2>
+            <p className="text-gray-500 mb-8">
+                Oops! The page you're looking for doesn't exist.
+            </p>
+        </div>
+    );
+}