Fix invoice ProjectName not pre-filling on edit; add to Details view

Edit GET now falls back to job.ProjectName for invoices created before the
column was added. Details view shows Project Name alongside Customer PO.

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

src/PowderCoating.Web/Controllers/InvoicesController.cs

@@ -903,7 +903,7 @@ public class InvoicesController : Controller
                 InternalNotes = invoice.InternalNotes,
                 Terms = invoice.Terms,
                 CustomerPO = invoice.CustomerPO,
-                ProjectName = invoice.ProjectName,
+                ProjectName = invoice.ProjectName ?? invoice.Job?.ProjectName,
                 InvoiceItems = invoice.InvoiceItems
                     .Where(i => !i.IsDeleted)
                     .OrderBy(i => i.DisplayOrder)