Fix: Replace extra </div> with </> for fragment closing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/learn/[unitId]/quiz/page.tsx

@@ -152,6 +152,6 @@ export default function QuizPage() {
           <p className={isDark ? 'text-white/60' : 'text-slate-500'}>Keine Quiz-Fragen verfuegbar.</p>
         )}
       </div>
-    </div>
+    </>
   )
 }