feat(cards): multiple-choice Card-Type mit dynamischen Distractors

- CardTypeSchema: 'multiple-choice' (Felder: front + answer, distractor_pool optional)
- subIndexCount: 'multiple-choice' → 1
- GET /api/v1/decks/:deckId/distractors: N zufällige Feldwerte anderer Karten
  im Deck; field-Allowlist (front/back/answer/question); RANDOM() ORDER; Fallback
  auf distractor_pool wenn Deck < 4 Karten
- fetchDistractors(): Frontend-Client-Funktion
- MultipleChoiceView.svelte: lädt Distractors on mount, shuffelt 4 Optionen,
  zeigt Sofort-Feedback (correct/wrong/neutral), Keyboard 1–4 + Space;
  auto-grade correct→good, wrong→again
- Study-Page: isMultipleChoice + multipleChoiceData derived, Action-Bar
  ausgeblendet, onKey delegiert

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/cards-domain/src/fsrs.ts

@@ -153,6 +153,8 @@ export function subIndexCount(type: string): number {
 			return 1;
 		case 'multiple-choice':
 			return 1;
+		case 'multiple-choice':
+			return 1;
 		case 'cloze':
 			throw new Error(
 				'subIndexCount("cloze") not supported — use subIndexCountForCloze(text) from @cards/domain'

packages/cards-domain/src/schemas/card.ts

@@ -12,6 +12,7 @@ export const CardTypeSchema = z.enum([
 	'image-occlusion',
 	'audio-front',
 	'typing',
+	'multiple-choice',
 ]);
 export type CardType = z.infer<typeof CardTypeSchema>;
 
@@ -50,7 +51,7 @@ export function validateFieldsForType(
 		'image-occlusion': ['image_ref', 'mask_regions'],
 		'audio-front': ['audio_ref', 'back'],
 		typing: ['front', 'answer'],
-		'multiple-choice': ['question', 'options', 'correct_index'],
+		'multiple-choice': ['front', 'answer'],
 	};
 	const need = required[type] ?? [];
 	const missing = need.filter((k) => !(k in fields));