refactor(landing): migrate to shared assets.21no.de design system

- Replace custom 710-line style.css with shared CSS from assets.21no.de
- Match 21no.de design tokens: #0a0a0b bg, #38bdf8 accent, Outfit + JetBrains Mono
- Add proper nav with 21no.de logo, stats row, 6-card feature grid
- Condensed from 1119 lines (48KB) to 385 lines (25KB)
- Preserve all SEO meta, JSON-LD, CSP, OG/Twitter tags
- Keep syntax-highlighted code examples and security badges

Author: molty3000