feat(a11y): add skip to content link

- Add SkipLink component that links to #main-content
- Add nav.skipToContent translation key
- Integrate SkipLink in App.tsx
- Add id="main-content" to main elements in pages
- Add test for SkipLink

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

src/components/ui/__tests__/SkipLink.test.tsx

@@ -0,0 +1,21 @@
+// @vitest-environment jsdom
+import { render, screen, cleanup } from '@testing-library/react';
+import { describe, it, expect, afterEach } from 'vitest';
+import { SkipLink } from '../SkipLink';
+import { LanguageProvider } from '../../../i18n';
+
+describe('SkipLink', () => {
+  afterEach(() => {
+    cleanup();
+  });
+
+  it('renders correctly', () => {
+    render(
+      <LanguageProvider>
+        <SkipLink />
+      </LanguageProvider>
+    );
+    const link = screen.getByRole('link');
+    expect(link.getAttribute('href')).toBe('#main-content');
+  });
+});