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/styles/index.css

@@ -254,6 +254,42 @@ input:focus, select:focus {
     display: flex;
 }
 
+/* Screen reader only content */
+.sr-only {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border: 0;
+}
+
+/* Focus styles for better accessibility */
+*:focus {
+    outline: 2px solid var(--color-primary);
+    outline-offset: 2px;
+}
+
+/* Skip link for keyboard navigation */
+.skip-link {
+    position: absolute;
+    top: -40px;
+    left: 6px;
+    background: var(--color-primary);
+    color: white;
+    padding: 8px;
+    text-decoration: none;
+    border-radius: 4px;
+    z-index: 10000;
+}
+
+.skip-link:focus {
+    top: 6px;
+}
+
 /* Responsive fullscreen toggle */
 @media screen and (max-width: 480px) {
     .fullscreenToggle {