chore: improve production deployment flexibility

- Switch frontend from nginx to caddy for consistency with Coolify
- Make VITE_API_URL optional, auto-derive from window.location.origin/api
- Simplifies deployment by not requiring build-time API URL

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -41,7 +41,7 @@ services:
       context: ./frontend
       dockerfile: Dockerfile.prod
       args:
-        VITE_API_URL: ${VITE_API_URL:?VITE_API_URL required}
+        VITE_API_URL: ${VITE_API_URL:-}
     ports:
       - "8080:8080"
     depends_on: