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.Web/Views/Invoices/Create.cshtml

@@ -175,11 +175,11 @@
                                 <a tabindex="0" class="help-icon" role="button"
                                    data-bs-toggle="popover" data-bs-placement="right" data-bs-trigger="focus"
                                    data-bs-title="Payment Terms"
-                                   data-bs-content="Free-text field that prints on the invoice (e.g., 'Net 30', 'Due on Receipt', '2% 10 Net 30'). Pre-filled from the customer's default payment terms. Changing it here only affects this invoice.">
+                                   data-bs-content="Prints on the invoice. Pre-filled from your App Defaults. Changing it here only affects this invoice.">
                                     <i class="bi bi-question-circle"></i>
                                 </a>
                             </div>
-                            <input asp-for="Terms" class="form-control" placeholder="e.g. Net 30" />
+                            <select asp-for="Terms" asp-items="ViewBag.PaymentTermsOptions" class="form-select"></select>
                         </div>
                     </div>
                 </div>
@@ -446,6 +446,7 @@
 </div>
 
 @section Scripts {
+<script src="~/js/invoice-due-date.js"></script>
 <script>
     let itemCount = @Model.InvoiceItems.Count;
     const merchandiseItems = @Html.Raw(ViewBag.MerchandiseItems ?? "[]");