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>

apps/landing/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "landing",
+  "version": "0.0.0",
+  "private": true,
+  "scripts": {
+    "dev": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview"
+  },
+  "dependencies": {
+    "astro": "^5.7.10",
+    "@astrojs/react": "^4.2.1",
+    "@tailwindcss/vite": "^4.1.4",
+    "tailwindcss": "^4.1.4",
+    "react": "^19.1.0",
+    "react-dom": "^19.1.0",
+    "react-icons": "^5.5.0"
+  }
+}