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/Modal.tsx

@@ -0,0 +1,36 @@
+import { h } from 'preact';
+import styles from './Modal.module.css';
+
+interface ModalProps {
+  open: boolean;
+  title: string;
+  message: string;
+  onCancel: () => void;
+  onConfirm: () => void;
+}
+
+/**
+ * Generic modal dialog for confirmation.
+ */
+const Modal = ({ open, title, message, onCancel, onConfirm }: ModalProps) => {
+  if (!open) return null;
+  return (
+    <div className={styles['modal'] + ' ' + styles['show']} role="dialog" aria-modal="true" aria-labelledby="modal-title">
+      <div className={styles['modal-content']}>
+        <div className={styles['modal-header']}>
+          <span className={styles['modal-title']} id="modal-title">{title}</span>
+          <button className={styles['close-button']} onClick={onCancel} aria-label="Schließen">×</button>
+        </div>
+        <div className={styles['modal-body']}>
+          <div className={styles['modal-message']}>{message}</div>
+        </div>
+        <div className={styles['modal-footer']}>
+          <button className={styles['modal-button'] + ' ' + styles['cancel']} onClick={onCancel} aria-label="Abbrechen">Abbrechen</button>
+          <button className={styles['modal-button'] + ' ' + styles['confirm']} onClick={onConfirm} aria-label="Löschen">Löschen</button>
+        </div>
+      </div>
+    </div>
+  );
+};
+
+export default Modal;