feat: redesign landing page with shared 21no.de design system

- Replace docsify shell with full-featured landing page
- Match 0http branding: near-black bg, sky-blue accent, Outfit + JetBrains Mono
- 8 shared CSS imports from assets.21no.de
- New sections: Hero with stats, Features grid (6 cards), Security defaults (6 items), Quick Start examples (6), Core Capabilities, Configuration table, Integrations (6), Footer
- Preserve all content from docs/README.md — nothing reduced
- Favicon, SEO meta tags, OG/Twitter cards, canonical URL
- Scroll-to-top button, scroll-triggered fade-in animations
- Responsive: collapses to single column at 768px

Author: molty3000