refactor(dashboard): consistent effective-user in disable-login warning; doc the config read

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardServiceCollectionExtensions.cs

@@ -22,7 +22,8 @@ public static class DashboardServiceCollectionExtensions
     /// <param name="services">Service collection to register services.</param>
     /// <param name="configuration">
     /// Application configuration, used to bind the shared LDAP provider's options
-    /// from the <c>MxGateway:Ldap</c> section.
+    /// from the <c>MxGateway:Ldap</c> section. Also read to select the dashboard
+    /// authentication scheme via the <c>MxGateway:Dashboard:DisableLogin</c> dev flag.
     /// </param>
     public static IServiceCollection AddGatewayDashboard(
         this IServiceCollection services,
@@ -80,7 +81,9 @@ public static class DashboardServiceCollectionExtensions
                         "DASHBOARD LOGIN DISABLED (MxGateway:Dashboard:DisableLogin=true) — every request is "
                         + "authenticated as '{User}' with full permissions ({Roles}). Dev/test only; never "
                         + "enable in production.",
-                        gatewayOptions.Dashboard.AutoLoginUser ?? DashboardAutoLoginAuthenticationHandler.DefaultUser,
+                        string.IsNullOrWhiteSpace(gatewayOptions.Dashboard.AutoLoginUser)
+                            ? DashboardAutoLoginAuthenticationHandler.DefaultUser
+                            : gatewayOptions.Dashboard.AutoLoginUser!.Trim(),
                         $"{DashboardRoles.Admin}, {DashboardRoles.Viewer}"));
         }
         else