move global 404 page to app router

client/src/app/[code]/not-found.tsx

@@ -1,8 +0,0 @@
-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>
-  );
-}