Lookup Users Under System Roles

Allow admins to select a system role and view all users currently assigned to it — consistent with how custom roles already work.

Current behavior:

• For custom roles, admins can select a specific role and see the list of users who have it enabled (forward + reverse lookup both work - see image #2 “stack manage”).

• For system roles, only the forward lookup is possible (user → role). There's no way to select a system role and see which users have it assigned as “System roles cannot be managed”.