🔒️(frontend) hide Nginx server version in error responses

Remove version disclosure in /assets/ error pages identified by security
auditor to prevent information leakage vulnerability.

src/frontend/default.conf

@@ -1,6 +1,7 @@
 server {
   listen 8080;
   server_name localhost;
+  server_tokens off;
 
   root /usr/share/nginx/html;