fix: use Array.isArray guard instead of unsafe cast in MultipleSelect

frontend/src/components/questions/MultipleSelect.tsx

@@ -15,7 +15,7 @@ export default function MultipleSelect({ q, value, onChange, locked, graded }: P
     if (locked) return
     onChange(current.includes(id) ? current.filter((v) => v !== id) : [...current, id])
   }
-  const answerSet = new Set((q.answer as string[]) ?? [])
+  const answerSet = new Set(Array.isArray(q.answer) ? q.answer : [])
 
   return (
     <div className="choices">