Refactor nginx configuration and update README

- Moved BACKEND_URL configuration to the frontend service in the README.
- Added validation for the generated nginx configuration in the entrypoint script.
- Removed fallback nginx configuration copy from the Dockerfile.
- Adjusted nginx template to ensure proper header formatting.

This improves the deployment process and clarifies configuration instructions.

README.md

@@ -122,12 +122,12 @@ When ExcaliDash runs behind Traefik, Nginx, or another reverse proxy, configure
 - `BACKEND_URL` (frontend) tells the Nginx container how to reach the backend from inside Docker/Kubernetes. Override it if your reverse proxy exposes the backend under a different hostname.
 
 ```yaml
-frontend:
-  environment:
-    - BACKEND_URL=excalidash-backend.default.svc.cluster.local:8000
 backend:
   environment:
     - FRONTEND_URL=https://excalidraw.example.com
+frontend:
+  environment:
+    - BACKEND_URL=excalidash-backend.default.svc.cluster.local:8000
 ```
 
 # Development