Add WarningPermanent toast type and upgrade invoice failure notifications

Email delivery failures and PDF generation errors now show a permanent
warning/error toast that requires manual dismissal, so users cannot
accidentally miss critical action-blocking feedback.

- ToastHelper: WarningPermanent TempData key + Warning/WarningPermanent
  extension methods on both ITempDataDictionary and Controller
- SetNotificationResultToast: NotificationStatus.Failed now uses
  ToastWarningPermanent (previously auto-dismissed in 5 s)
- InvoicesController.Send: TempData["Warning"] → TempData["WarningPermanent"]
  when PDF generation or email dispatch fails
- InvoicesController.DownloadPdf: TempData["Error"] → TempData["ErrorPermanent"]
  with the actual exception message so root cause is visible
- _Layout.cshtml: WarningPermanent hidden div
- toast-notifications.js: WarningPermanent handler (timeOut: 0)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Helpers/ToastHelper.cs

@@ -31,6 +31,9 @@ namespace PowderCoating.Web.Helpers
         /// <summary>TempData key read by the layout to render a yellow warning toast.</summary>
         private const string WarningKey = "Warning";
 
+        /// <summary>TempData key read by the layout to render a yellow warning toast that does not auto-dismiss.</summary>
+        private const string WarningPermanentKey = "WarningPermanent";
+
         /// <summary>TempData key read by the layout to render a blue info toast.</summary>
         private const string InfoKey = "Info";
 
@@ -67,6 +70,15 @@ namespace PowderCoating.Web.Helpers
             tempData[WarningKey] = message;
         }
 
+        /// <summary>
+        /// Stores a warning (yellow) toast that requires manual dismissal (no auto-timeout).
+        /// Use for critical warnings the user must not miss, such as email delivery failures.
+        /// </summary>
+        public static void WarningPermanent(this ITempDataDictionary tempData, string message)
+        {
+            tempData[WarningPermanentKey] = message;
+        }
+
         /// <summary>
         /// Stores an informational (blue) toast message in TempData for display on
         /// the next page render after a redirect.
@@ -123,6 +135,15 @@ namespace PowderCoating.Web.Helpers
             controller.TempData.Warning(message);
         }
 
+        /// <summary>
+        /// Stores a permanent warning (yellow, no auto-dismiss) in the controller's TempData.
+        /// Use for failures the user must not miss — email delivery errors, PDF generation failures.
+        /// </summary>
+        public static void ToastWarningPermanent(this Controller controller, string message)
+        {
+            controller.TempData.WarningPermanent(message);
+        }
+
         /// <summary>
         /// Stores an informational (blue) toast in the controller's TempData.
         /// Convenience wrapper around <see cref="ToastHelper.Info"/>.
@@ -165,7 +186,7 @@ namespace PowderCoating.Web.Helpers
                         : $"{channel} notification was skipped.");
                     break;
                 case NotificationStatus.Failed:
-                    controller.ToastWarning(!string.IsNullOrEmpty(log.ErrorMessage)
+                    controller.ToastWarningPermanent(!string.IsNullOrEmpty(log.ErrorMessage)
                         ? $"{channel} delivery failed: {log.ErrorMessage}"
                         : $"{channel} notification failed.");
                     break;