feat(a11y): add skip to content link

- Add `SkipLink` component to `src/components/ui`
- Integrate `SkipLink` in `src/App.tsx`
- Add `id="main-content"` and proper focus management to page containers
- Add translations for the skip link text
- Add unit test for `SkipLink`

Co-authored-by: ragusa-it <196988693+ragusa-it@users.noreply.github.com>

src/components/ui/SkipLink.tsx

@@ -0,0 +1,12 @@
+import { useTranslation } from '../../i18n';
+import styles from './SkipLink.module.css';
+
+export function SkipLink() {
+  const { t } = useTranslation();
+
+  return (
+    <a href="#main-content" className={styles.skipLink}>
+      {t.nav.skipToContent}
+    </a>
+  );
+}