Fix: ManageUsers component missing useAuth() - caused blank admin page

The ManageUsers sub-component referenced user?.role for conditional
role dropdown options but didn't call useAuth() to get the user object.
This caused 'user is not defined' JS errors and blank admin pages.

Browser E2E: 20/20 tests pass.

docker/nginx/default.conf

@@ -3,7 +3,7 @@
 # /*     → frontend static files
 
 upstream backend_api {
-    server backend:3001;
+    server backend:3004;
     keepalive 16;
 }