feat(cards): audio-front Card-Type

- CardTypeSchema: 'audio-front' als vollwertiger Type (fields: audio_ref + back + front?)
- subIndexCount: 'audio-front' → 1
- AudioFrontView.svelte: custom Play/Pause-Button, audio via /api/v1/media/:id,
  optionaler Hint-Text; Antwort-Markdown läuft über bestehenden answerHtml-Pfad
- Study-Page: isAudioFront + audioFrontData derived, AudioFrontView eingebunden

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

packages/cards-domain/src/fsrs.ts

@@ -147,7 +147,7 @@ export function subIndexCount(type: string): number {
 			throw new Error(
 				'subIndexCount("image-occlusion") not supported — use maskRegionCount(fields.mask_regions) from @cards/domain'
 			);
-		case 'audio':
+		case 'audio-front':
 			return 1;
 		case 'multiple-choice':
 			return 1;

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

@@ -10,6 +10,7 @@ export const CardTypeSchema = z.enum([
 	'basic-reverse',
 	'cloze',
 	'image-occlusion',
+	'audio-front',
 ]);
 export type CardType = z.infer<typeof CardTypeSchema>;
 
@@ -20,7 +21,7 @@ export const CardTypeFutureSchema = z.enum([
 	'cloze',
 	'type-in',
 	'image-occlusion',
-	'audio',
+	'audio-front',
 	'multiple-choice',
 ]);
 
@@ -46,7 +47,7 @@ export function validateFieldsForType(
 		cloze: ['text'],
 		'type-in': ['question', 'expected'],
 		'image-occlusion': ['image_ref', 'mask_regions'],
-		audio: ['audio_ref'],
+		'audio-front': ['audio_ref', 'back'],
 		'multiple-choice': ['question', 'options', 'correct_index'],
 	};
 	const need = required[type] ?? [];