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/project.json

@@ -1,24 +1,12 @@
 {
   "name": "landing",
   "$schema": "../../node_modules/nx/schemas/project-schema.json",
-  "sourceRoot": "apps/landing",
+  "sourceRoot": "apps/landing/src",
   "projectType": "application",
   "tags": [],
-  "// targets": "to see all targets run: nx show project landing --web",
   "targets": {
-    "serve": {
-      "executor": "@nx/next:server",
-      "options": {
-        "buildTarget": "landing:build",
-        "dev": true
-      }
-    },
-    "build": {
-      "executor": "@nx/next:build",
-      "outputs": ["{options.outputPath}"],
-      "options": {
-        "outputPath": "dist/apps/landing"
-      }
-    }
+    "dev": { "command": "cd apps/landing && npx astro dev" },
+    "build": { "command": "cd apps/landing && npx astro build" },
+    "preview": { "command": "cd apps/landing && npx astro preview" }
   }
 }