feat(frontend): add 404 page for missing job

Author: PenguinDevs

frontend/src/app/jobs/[id]/not-found.tsx

@@ -0,0 +1,20 @@
+import Image from "next/image";
+import Link from "next/link";
+
+export default function JobNotFound() {
+  return (
+    <div className="flex min-h-[80vh] flex-col items-center justify-center gap-6 px-4 text-center">
+      <Image src="/mac.svg" alt="MAC mascot" width={160} height={160} />
+      <h1 className="text-4xl font-bold text-white">404</h1>
+      <p className="text-lg text-white/50">
+        This job post is no longer taking applications or it does not exist.
+      </p>
+      <Link
+        href="/jobs"
+        className="rounded-lg bg-[#ffe22f] px-6 py-3 font-semibold text-black transition-opacity hover:opacity-80"
+      >
+        Browse jobs
+      </Link>
+    </div>
+  );
+}