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/Controllers/InvoicesController.cs

@@ -917,7 +917,7 @@ public class InvoicesController : Controller
 
             TempData["Success"] = $"Invoice {invoice.InvoiceNumber} marked as sent.";
             if (!pdfAndNotifSucceeded)
-                TempData["Warning"] = "The invoice is marked as sent, but PDF generation or the customer email failed. Check the notification logs or your email configuration.";
+                TempData["WarningPermanent"] = "The invoice is marked as sent, but PDF generation or the customer email failed. Check the notification logs or your email configuration.";
             return RedirectToAction(nameof(Details), new { id });
         }
         catch (Exception ex)
@@ -1302,8 +1302,8 @@ public class InvoicesController : Controller
         }
         catch (Exception ex)
         {
-            _logger.LogError(ex, "Error generating PDF for invoice {Id}", id);
-            TempData["Error"] = "An error occurred while generating the PDF.";
+            _logger.LogError(ex, "Error generating PDF for invoice {Id}. Inner: {Inner}", id, ex.InnerException?.Message ?? ex.Message);
+            TempData["ErrorPermanent"] = $"PDF generation failed: {ex.Message}";
             return RedirectToAction(nameof(Details), new { id });
         }
     }