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/wwwroot/js/toast-notifications.js

@@ -118,6 +118,12 @@ function displayTempDataMessages() {
         showWarning(warningMsg.textContent.trim());
     }
 
+    // Permanent warning — no auto-dismiss
+    const warningPerm = document.getElementById('tempdata-warning-permanent-message');
+    if (warningPerm && warningPerm.textContent.trim()) {
+        toastr.warning(warningPerm.textContent.trim(), 'Warning', { timeOut: 0, extendedTimeOut: 0 });
+    }
+
     // Info message
     const infoMsg = document.getElementById('tempdata-info-message');
     if (infoMsg && infoMsg.textContent.trim()) {