Fix: Quiz fallback to QA data when MC not generated

Vocab units created via /vocabulary/units only have QA items, no
pre-generated MC questions. Quiz now falls back to generating MC
questions client-side from QA items (EN word → 4 DE options).

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

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

@@ -42,10 +42,37 @@ export default function QuizPage() {
   const loadMC = async () => {
     setIsLoading(true)
     try {
-      const resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/mc`)
-      if (!resp.ok) throw new Error(`HTTP ${resp.status}`)
+      // Try MC endpoint first
+      let resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/mc`)
+      if (resp.ok) {
         const data = await resp.json()
         setQuestions(data.questions || [])
+        return
+      }
+
+      // Fallback: Generate MC from QA items (vocab units don't have pre-generated MC)
+      resp = await fetch(`${getApiBase()}/api/learning-units/${unitId}/qa`)
+      if (!resp.ok) throw new Error('Keine Daten verfuegbar')
+      const qaData = await resp.json()
+      const qaItems = qaData.qa_items || []
+      if (qaItems.length < 4) throw new Error('Zu wenige Woerter fuer Quiz (min. 4)')
+
+      // Build MC questions from QA: question=EN word, options=4 DE answers (1 correct + 3 random)
+      const generated: MCQuestion[] = qaItems.map((item: any, idx: number) => {
+        const others = qaItems.filter((_: any, i: number) => i !== idx)
+        const distractors = [...others].sort(() => Math.random() - 0.5).slice(0, 3)
+        const options = [
+          { id: item.id, text: item.answer },
+          ...distractors.map((d: any) => ({ id: d.id, text: d.answer })),
+        ].sort(() => Math.random() - 0.5)
+        return {
+          id: item.id,
+          question: item.question,
+          options,
+          correct_answer: item.id,
+        }
+      })
+      setQuestions(generated)
     } catch (err: any) {
       setError(err.message)
     } finally {