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/FullscreenToggle.tsx

@@ -0,0 +1,35 @@
+import { h } from 'preact';
+import { useCallback } from 'preact/hooks';
+import styles from './FullscreenToggle.module.css';
+
+/**
+ * Button to toggle fullscreen mode.
+ * @returns {import('preact').VNode}
+ */
+const FullscreenToggle = () => {
+  // Toggle fullscreen mode for the document
+  const handleToggle = useCallback(() => {
+    if (!document.fullscreenElement) {
+      document.documentElement.requestFullscreen();
+    } else {
+      document.exitFullscreen();
+    }
+  }, []);
+
+  return (
+    <button
+      id="fullscreen-toggle"
+      className={styles.fullscreenToggle}
+      onClick={handleToggle}
+      title="Vollbild umschalten"
+      aria-label="Vollbild umschalten"
+      type="button"
+    >
+      <svg viewBox="0 0 24 24" width="24" height="24" aria-hidden="true">
+        <path fill="currentColor" d="M7 14H5v5h5v-2H7v-3zm-2-4h2V7h3V5H5v5zm12 7h-3v2h5v-5h-2v3zM14 5v2h3v3h2V5h-5z"/>
+      </svg>
+    </button>
+  );
+};
+
+export default FullscreenToggle;