move global 404 page to app router

client/src/app/not-found.tsx

@@ -0,0 +1,8 @@
+export default function NotFound() {
+  return (
+    <div className="flex flex-col items-center justify-center gap-4">
+      <div className="text-2xl font-bold">Error 404</div>
+      <div className="text-xl">Not found</div>
+    </div>
+  );
+}