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/Interfaces/IPdfService.cs

@@ -25,6 +25,12 @@ public interface IPdfService
         CompanyInfoDto companyInfo,
         QuoteTemplateSettingsDto? template = null);
 
+    Task<byte[]> GeneratePackingSlipPdfAsync(
+        InvoiceDto invoiceDto,
+        byte[]? companyLogo,
+        string? companyLogoContentType,
+        CompanyInfoDto companyInfo);
+
     Task<byte[]> GeneratePurchaseOrderPdfAsync(
         PurchaseOrderDto po,
         byte[]? companyLogo,