Enhance user management: prevent deletion of the last admin user and restrict admin type changes for the last admin account

Co-authored-by: Copilot <copilot@github.com>

FoodsharingSiegen.Server/Pages/Users.razor.cs

@@ -57,6 +57,11 @@ namespace FoodsharingSiegen.Server.Pages
         /// </summary>
         private bool IsEditing { get; set; }
 
+        /// <summary>
+        ///     Gets a value indicating whether the current editing user is the last admin
+        /// </summary>
+        private bool IsLastAdmin => IsEditing && EditModel?.Type == UserType.Admin && UserList?.Count(x => x.Type == UserType.Admin) <= 1;
+
         /// <summary>
         ///     Gets the value of the user groups (ab)
         /// </summary>
@@ -184,12 +189,6 @@ namespace FoodsharingSiegen.Server.Pages
         /// <returns>A task that represents the asynchronous remove operation.</returns>
         private async Task RemoveUserAsync(User user)
         {
-            if (user.IsAdmin())
-            {
-                await Notification.Error("Admins können nicht gelöscht werden!");
-                return;
-            }
-
             await ConfirmDialog.ShowAsync(ModalService, "Bestätigen", $"User {user.Mail} löschen?", async () =>
             {
                 var removeR = await UserService.RemoveAsync(user.Id);