Add full Progressive Web App support to The Nexus:
- manifest.json: Web app manifest with Nexus branding, icons, and PWA config
- sw.js: Service worker with cache-first strategy for local assets and
stale-while-revalidate for CDN resources (Three.js, fonts)
- offline.html: Styled offline fallback page with auto-reconnect
- icons/: SVG icons (standard + maskable) with crystalline Nexus design
- index.html: Add manifest link, theme colors, Apple PWA meta tags,
and service worker registration script
Features:
- Works offline after first visit
- Installable to home screen on mobile and desktop
- Graceful degradation when offline
- Auto-refresh when connection restored
Fixes#14
