feat: make SITE_URL and SITE_DOMAIN dynamic via .env and docker compose

- Create .env and .env.example with SITE_DOMAIN=riverrooks.dev and SITE_URL=https://riverrooks.dev
- Migrate src/_data/site.json to src/_data/site.js reading environment variables dynamically
- Update subdomains across Forgejo, CyberChef, Stirling PDF, and Uptime Kuma to inherit site.domain
- Update docker-compose.yml and Dockerfile to accept dynamic SITE_URL and SITE_DOMAIN build args
- Add .env to .gitignore

src/_includes/partials/footer.njk

@@ -8,6 +8,6 @@
 
   <div class="footer-right">
     <span>&copy; {{ currentYear or "2026" }} {{ site.author }}</span>
-    <a href="https://git.rooks.dev" target="_blank" rel="noopener noreferrer" class="footer-link">Source Forge ↗</a>
+    <a href="https://git.{{ site.domain }}" target="_blank" rel="noopener noreferrer" class="footer-link">Source Forge ↗</a>
   </div>
 </footer>