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.Core/Entities/Invoice.cs

@@ -48,6 +48,7 @@ public class Invoice : BaseEntity
     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 (e.g., 2 means 2% discount).