Add packing slip PDF to invoice details page

Generates a no-price packing slip (items, color, qty + signature line) via
QuestPDF. New DownloadPackingSlip action reuses existing invoice data pipeline;
Packing Slip button opens inline in a new tab same as Print/PDF.

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

src/PowderCoating.Application/DTOs/Invoice/InvoiceDtos.cs

@@ -33,6 +33,10 @@ public class InvoiceDto
     public string? CustomerEmail { get; set; }
     public string? CustomerPhone { get; set; }
     public string? CustomerMobilePhone { get; set; }
+    public string? CustomerAddress { get; set; }
+    public string? CustomerCity { get; set; }
+    public string? CustomerState { get; set; }
+    public string? CustomerZipCode { get; set; }
     public bool CustomerNotifyByEmail { get; set; }
     public bool CustomerNotifyBySms { get; set; }
     public string? PreparedById { get; set; }