add challenge completed alert to fall-2025

Author: adiati98

src/app/monthlychallenges/(challenges)/fall-2025/page.tsx

@@ -19,6 +19,11 @@ export const metadata = handle.meta;
 export default function Challenge() {
 	return (
 		<>
+			<div className="alert alert-success">
+				This monthly challenge is complete. Congratulations! Please join us for
+				the <Link href="/monthlychallenges/nov-2025">next challenge</Link>!
+			</div>
+
 			<h1>
 				<small>Bi-Monthly Challenge for Fall 2025:</small> Preptember &
 				Hacktoberfest!