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.Web/Views/Invoices/Details.cshtml

@@ -51,6 +51,10 @@
             <a asp-action="DownloadPdf" asp-route-id="@Model.Id" class="btn btn-outline-secondary">
                 <i class="bi bi-file-pdf me-2"></i>PDF
             </a>
+            <a asp-action="DownloadPackingSlip" asp-route-id="@Model.Id" asp-route-inline="true"
+               class="btn btn-outline-secondary" target="_blank" rel="noopener" title="Print Packing Slip">
+                <i class="bi bi-box-seam me-2"></i>Packing Slip
+            </a>
             <a asp-action="Index" class="btn btn-outline-secondary">
                 <i class="bi bi-arrow-left me-2"></i>Back
             </a>