feat(study): Multiple-Choice-Karten gerendert

CardRenderer für multipleChoice ist nicht mehr Placeholder. Web-
Vorbild: MultipleChoiceView.svelte.

MultipleChoiceCardView (Features/Study/):
- Lädt Distractors vom Server beim card.id-Wechsel
  (CardsAPI.distractors(deckId, cardId, field, count))
- Versucht erst field=answer, fallback field=back (für Decks mit
  basic/basic-reverse-Karten daneben)
- Fallback auf distractor_pool-Feld (newline-separated) wenn
  Deck zu klein
- 4 Optionen shuffled = [answer + 3 Distractors]
- User-Tap markiert Auswahl (kein erneutes Pick möglich)
- Vor Flip: nur Selected-Hint (primary border)
- Nach Flip: richtige = green-check, falsche-gewählte = red-cross,
  unselected richtige bleibt green-highlight
- Fallback "tooFew" (< 1 Distractor): zeigt Antwort nach Flip
  ohne Auswahl

CardsAPI.distractors → DistractorsResponse {distractors: [String]}.

Typing bleibt Placeholder — eigene UI-Pattern (Text-Input + Diff)
brauchen mehr Design-Arbeit, separate Phase.

Build 7 → 8, 35 Tests grün.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Sources/Core/API/CardsAPI.swift

@@ -52,6 +52,21 @@ actor CardsAPI {
         return try decoder.decode(CardListResponse.self, from: data).cards
     }
 
+    /// `GET /api/v1/decks/:deckId/distractors` — N zufällige Feldwerte
+    /// aus anderen Karten desselben Decks. Server-Schema erlaubt nur
+    /// `front`, `back`, `answer`, `question` als field.
+    func distractors(
+        deckId: String,
+        cardId: String,
+        field: String = "answer",
+        count: Int = 3
+    ) async throws -> [String] {
+        let path = "/api/v1/decks/\(deckId)/distractors?card_id=\(cardId)&field=\(field)&count=\(count)"
+        let (data, http) = try await transport.request(path: path)
+        try ensureOK(http, data: data)
+        return try decoder.decode(DistractorsResponse.self, from: data).distractors
+    }
+
     /// `GET /api/v1/reviews/due?deck_id=...&limit=500` — Anzahl fälliger
     /// Reviews in einem Deck.
     func dueCount(deckId: String) async throws -> Int {

Sources/Core/Domain/Deck.swift

@@ -129,3 +129,8 @@ struct CardListResponse: Decodable, Sendable {
 struct DueReviewsResponse: Decodable, Sendable {
     let total: Int
 }
+
+/// Server-Response von `GET /api/v1/decks/:deckId/distractors`.
+struct DistractorsResponse: Decodable, Sendable {
+    let distractors: [String]
+}