feat: migrate landing page from Next.js to Astro

Replaces the heavy Next.js SSR app with Astro static site:
- Zero JS shipped for static pages (hero, community, CTA, footer)
- Server-side data fetching in Astro frontmatter (events, hackathon, testimonials)
- Only 4 React islands for interactive parts (mobile menu, roadmap voting, testimonials home, feature request)
- Google Fonts via <link> instead of next/font
- Nix build changed from Node.js systemd service to static nginx (mkStaticAppModule)
- npmDepsHash needs update (set to fake hash, CI will provide correct one)

Performance gains:
- No Node.js server process needed (was using ~10MB RAM)
- No Next.js runtime JS (~200KB+ saved)
- No framer-motion bundle (~130KB saved)
- Static HTML served directly by nginx

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -37,15 +37,18 @@
   "private": true,
   "dependencies": {
     "@ant-design/icons": "^6.1.1",
+    "@astrojs/react": "^5.0.3",
     "@hookform/resolvers": "^5.2.2",
     "@iconify/react": "^6.0.2",
     "@marsidev/react-turnstile": "^1.5.0",
     "@radix-ui/react-label": "^2.1.8",
     "@radix-ui/react-slot": "^1.2.4",
     "@redocly/ajv": "^8.18.1",
+    "@tailwindcss/vite": "^4.2.2",
     "@tanstack/react-query": "^5.95.2",
     "@tanstack/react-store": "^0.9.3",
     "@tanstack/react-table": "^8.21.3",
+    "astro": "^6.1.5",
     "axios": "^1.14.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",