feat(storage): migrate to IndexedDB with localStorage fallback and async app flow

- Add IndexedDB service with schema, indexes, and player stats
- Migrate GameService to async IndexedDB and auto-migrate from localStorage
- Update hooks and App handlers to async; add error handling and UX feedback
- Convert remaining JSX components to TSX
- Add test utility for IndexedDB and migration checks
- Extend game types with sync fields for future online sync

src/components/GameCompletionModal.tsx

@@ -0,0 +1,63 @@
+import { h } from 'preact';
+import modalStyles from './Modal.module.css';
+import styles from './GameCompletionModal.module.css';
+import type { Game } from '../types/game';
+
+interface GameCompletionModalProps {
+  open: boolean;
+  game: Game | null;
+  onConfirm: () => void;
+  onClose: () => void;
+  onRematch: () => void;
+}
+
+/**
+ * Modal shown when a game is completed.
+ */
+const GameCompletionModal = ({ open, game, onConfirm, onClose, onRematch }: GameCompletionModalProps) => {
+  if (!open || !game) return null;
+
+  const playerNames = [game.player1, game.player2, game.player3].filter(Boolean);
+  const scores = [game.score1, game.score2, game.score3].filter((_, i) => playerNames[i]);
+  let maxScore, winners, winnerText;
+
+  if (game.forfeitedBy) {
+    winnerText = `${game.winner} hat gewonnen, da ${game.forfeitedBy} aufgegeben hat.`;
+  } else {
+    maxScore = Math.max(...scores);
+    winners = playerNames.filter((name, idx) => scores[idx] === maxScore);
+    winnerText = winners.length > 1
+      ? `Unentschieden zwischen ${winners.join(' und ')}`
+      : `${winners[0]} hat gewonnen!`;
+  }
+  
+  return (
+    <div id="game-completion-modal" className={modalStyles['modal'] + ' ' + modalStyles['show']} role="dialog" aria-modal="true" aria-labelledby="completion-modal-title">
+      <div className={modalStyles['modal-content']}>
+        <div className={modalStyles['modal-header']}>
+          <span className={modalStyles['modal-title']} id="completion-modal-title">Spiel beendet</span>
+          <button className={modalStyles['close-button']} onClick={onClose} aria-label="Schließen">×</button>
+        </div>
+        <div className={modalStyles['modal-body']}>
+          <div className={styles['final-scores']}>
+            {playerNames.map((name, idx) => (
+              <div className={styles['final-score']} key={name + idx}>
+                <span className={styles['player-name']}>{name}</span>
+                <span className={styles['score']}>{scores[idx]}</span>
+              </div>
+            ))}
+          </div>
+          <div className={styles['winner-announcement']}><h3>{winnerText}</h3></div>
+
+        </div>
+        <div className={modalStyles['modal-footer']}>
+          <button className={styles['btn'] + ' ' + styles['btn--warning']} onClick={onConfirm} aria-label="Bestätigen">Bestätigen</button>
+          <button className={styles['btn'] + ' ' + styles['btn--primary']} onClick={onRematch} aria-label="Rematch">Rematch</button>
+          <button className={styles['btn']} onClick={onClose} aria-label="Abbrechen">Abbrechen</button>
+        </div>
+      </div>
+    </div>
+  );
+};
+
+export default GameCompletionModal;