fix(proxy): remove double slash in nginx API proxy_pass

API_URL already includes a trailing slash, so the extra slash in
proxy_pass produced //auth/login paths, causing 404s. Also clear
ROOT_PATH since nginx strips /api/ before proxying.

frontend/nginx.conf.template

@@ -15,7 +15,7 @@ server {
 
     # Proxy API requests to backend
     location /api/ {
-        proxy_pass ${API_URL}/;
+        proxy_pass ${API_URL};
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';