Add admin user management and password-change flow

Introduce full admin user listing/detail endpoints and a forced password-change flow. Backend: make CurrentUserResponse.UserName nullable and add ToCurrentUserResponseAsync extension; AppUserController now exposes GET /auth/user (list) and GET /auth/user/{id} (detail) using UserManager and Admin-only policy; AuthController uses the new mapper and after successful password change clears MustChangePassword, updates UpdatedAt and persists changes (with error handling) before updating security stamp. Frontend: add admin users pages (list + detail), ChangePassword page and route, adminUsers and enhanced authSession services (typed responses, changePassword API, error mapping), router guard to redirect users with mustChangePassword=true to the change-password flow, and show success banner on login after password change. UI tweaks: separate admin section in sidebar, add password-change entries in account menu, footer sizing fixes, and various layout/UX improvements. These changes enable admin account management and enforce secure password updates across the app.

API/Contracts/Auth/CurrentUserResponse.cs

@@ -3,7 +3,7 @@
     public class CurrentUserResponse
     {
         public Guid Id { get; set; }
-        public string UserName { get; set; } = string.Empty;
+        public string? UserName { get; set; } = string.Empty;
         public List<string> Roles { get; set; } = new();
         public bool IsActive { get; set; }
         public bool MustChangePassword { get; set; }