feat(a11y): add accessible Skip to Content link

- Added `SkipLink` component that is visible on focus.
- Updated `App.tsx` to include `SkipLink` and wrap content in `<main id="main-content">`.
- Added translation keys for skip link in English and German.
- Added unit tests for `SkipLink`.

This improves accessibility for keyboard and screen reader users by allowing them to bypass the navigation menu.

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

src/i18n/de.ts

@@ -1,6 +1,7 @@
 export const de = {
   // Navigation
   nav: {
+    skipLink: 'Zum Inhalt springen',
     home: 'Startseite',
     about: 'Über uns',
     contact: 'Kontakt',