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/src/styles/globals.css

@@ -1,6 +1,4 @@
 @import 'tailwindcss';
-@import 'tw-animate-css';
-@source "../../../../libs/ui/src/**/*.{ts,tsx}";
 
 @custom-variant dark (&:is(.dark *));
 
@@ -127,3 +125,12 @@
     @apply bg-background text-foreground;
   }
 }
+
+@keyframes fadeInUp {
+  from { opacity: 0; transform: translateY(20px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+@keyframes fadeIn {
+  from { opacity: 0; }
+  to { opacity: 1; }
+}