Fix invoice re-creation after void; add payment terms selector and shop supplies line

- Voided invoices no longer block creating a new invoice for the same job: voided invoice's
  JobId FK is cleared so the unique index slot is freed for the replacement
- Invoice Details view shows voided invoices as history rather than hiding them
- Payment terms: standardized SelectList (Due on Receipt, Net 15/30/45/60/90, 2% 10 Net 30,
  COD) with custom-term preservation; invoice-due-date.js auto-updates Due Date on term change
- Shop supplies on direct (no-quote) jobs: InvoicesController derives the shop supplies line
  from the company rate when the job has no source quote to read the pre-agreed amount from
- Job entity: ShopSuppliesAmount + ShopSuppliesPercent fields preserved through job lifecycle
- Migration: AddShopSuppliesAmountToJob

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

src/PowderCoating.Core/Entities/Job.cs

@@ -28,6 +28,8 @@ public class Job : BaseEntity
     // Pricing
     public decimal QuotedPrice { get; set; }
     public decimal FinalPrice { get; set; }
+    public decimal ShopSuppliesAmount { get; set; }
+    public decimal ShopSuppliesPercent { get; set; }
 
     // Discount & rush (mirrors quote fields; preserved through quote→job conversion and job edits)
     public bool IsRushJob { get; set; }