Add Project Name field to invoice create and edit forms

Stores ProjectName on the Invoice entity (previously only inherited from the
linked job at display time). Pre-fills from the job when creating from a job.
Migration: AddInvoiceProjectName.

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

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

@@ -89,6 +89,7 @@ public class CreateInvoiceDto
     public string? InternalNotes { get; set; }
     public string? Terms { get; set; }
     public string? CustomerPO { get; set; }
+    public string? ProjectName { get; set; }
     /// <summary>Early-payment discount percentage parsed from the customer's payment terms (e.g., 2.0 for "2/10 Net 30"). Informational — does not auto-apply.</summary>
     public decimal EarlyPaymentDiscountPercent { get; set; }
     /// <summary>Number of days within which the early-payment discount applies (e.g., 10 for "2/10 Net 30").</summary>
@@ -106,6 +107,7 @@ public class UpdateInvoiceDto
     public string? InternalNotes { get; set; }
     public string? Terms { get; set; }
     public string? CustomerPO { get; set; }
+    public string? ProjectName { get; set; }
     public List<CreateInvoiceItemDto> InvoiceItems { get; set; } = new();
 }