feat: add accessible Skip to Content link

- Add SkipLink component with off-screen styling
- Add translation keys for skip link text
- Integrate SkipLink in App.tsx and wrap content in #main-content
- Add unit tests for SkipLink

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

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

@@ -0,0 +1,34 @@
+// @vitest-environment jsdom
+import { render, screen, cleanup } from '@testing-library/react';
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { SkipLink } from '../SkipLink';
+
+// Mock i18n
+vi.mock('../../../i18n', () => ({
+  useTranslation: () => ({
+    t: {
+      nav: {
+        skipToContent: 'Skip to content',
+      },
+    },
+  }),
+}));
+
+describe('SkipLink', () => {
+  afterEach(() => {
+    cleanup();
+  });
+
+  it('renders a link with correct href and text', () => {
+    render(<SkipLink />);
+    const link = screen.getByRole('link', { name: /skip to content/i });
+    expect(link).toBeTruthy();
+    expect(link.getAttribute('href')).toBe('#main-content');
+  });
+
+  it('has class attribute for styling', () => {
+    render(<SkipLink />);
+    const link = screen.getByRole('link');
+    expect(link.getAttribute('class')).toBeTruthy();
+  });
+});