Fix accessibility gaps: skip link, semantic buttons, reduced motion, focus styles

[lukeinglis]

What

Fix 4 confirmed accessibility gaps:

1. Add skip-to-content link (WCAG 2.4.1 Level A)
2. Change scroll-to-top <div> to <button> (keyboard accessible)
3. Add prefers-reduced-motion support
4. Add :focus-visible outline styles

Factory Experiment: #6