Fix skip link visibility - hide until keyboard focus

Bug: Skip link was visible on page load instead of hidden until Tab focus.

Fix: Add proper CSS for .skip-link class:
- Position absolute with top: -40px (visually hidden)
- On focus: top: 0 (slides into view)
- Ubuntu orange background with white text
- High z-index to appear above content

This implements the standard WCAG 2.1 skip link pattern for keyboard navigation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: SolidRhino

src/index.css

@@ -21,6 +21,23 @@ code {
     monospace;
 }
 
+/* Skip link for keyboard navigation */
+.skip-link {
+  position: absolute;
+  top: -40px;
+  left: 0;
+  background: var(--ubuntu-orange);
+  color: white;
+  padding: 8px 16px;
+  text-decoration: none;
+  border-radius: 0 0 4px 0;
+  z-index: 100;
+}
+
+.skip-link:focus {
+  top: 0;
+}
+
 /* Focus indicators for accessibility */
 *:focus {
   outline: 3px solid var(--ubuntu-orange);