feat: cyberpunk theme + WCAG AAA accessibility (#6)

Theme unification with CorvidLabs cyberpunk aesthetic:
- Replace purple/blue gradient palette with cyan/magenta/green
- Sharpen border-radius (12-16px → 4-6px) for pixel aesthetic
- Add scanline overlay to hero section
- Use JetBrains Mono as primary font (monospace-first)
- Update all rgba() glow/shadow colors to match new palette

WCAG AAA accessibility:
- Add skip-navigation link
- Add <main id="main-content"> landmark
- Add aria-label="Main navigation" to nav
- Add :focus-visible styles with cyan outline
- Add prefers-reduced-motion media query
- Bump --text-dim to #a8abbd (8.7:1 contrast ratio on dark bg)
- All text colors now meet WCAG AAA 7:1 minimum contrast

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: corvid-agent