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/ValidationModal.tsx

@@ -0,0 +1,33 @@
+import { h } from 'preact';
+import styles from './Modal.module.css';
+
+interface ValidationModalProps {
+  open: boolean;
+  message: string;
+  onClose: () => void;
+}
+
+/**
+ * Modal for displaying validation errors.
+ */
+const ValidationModal = ({ open, message, onClose }: ValidationModalProps) => {
+  if (!open) return null;
+  return (
+    <div className={styles['modal'] + ' ' + styles['show']} id="validation-modal" role="alertdialog" aria-modal="true" aria-labelledby="validation-modal-title">
+      <div className={styles['modal-content']}>
+        <div className={styles['modal-header']}>
+          <span className={styles['modal-title']} id="validation-modal-title">Fehler</span>
+          <button className={styles['close-button']} onClick={onClose} 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={onClose} aria-label="OK">OK</button>
+        </div>
+      </div>
+    </div>
+  );
+};
+
+export default ValidationModal;