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/wwwroot/js/invoice-due-date.js

@@ -0,0 +1,56 @@
+/**
+ * Auto-calculates invoice Due Date from Invoice Date + Payment Terms.
+ * Parses common terms formats: "Net 30", "N/15", "Due on Receipt", "COD", "2% 10 Net 30".
+ * Only fires when Terms or Invoice Date changes; user can always override the Due Date field.
+ */
+(function () {
+    /// <summary>
+    /// Extracts the net payment days from a free-text terms string.
+    /// Returns null when the string can't be parsed (due date is left unchanged).
+    /// </summary>
+    function parseDays(terms) {
+        if (!terms || !terms.trim()) return null;
+        const t = terms.trim().toLowerCase();
+        if (/\b(receipt|due\s*now|cod|immediate)\b/.test(t)) return 0;
+        // "Net N" or "N/N" (e.g., "2% 10 Net 30" or "N/30")
+        let m = t.match(/\bnet\s+(\d+)/) || t.match(/\bn\/(\d+)/);
+        if (m) return parseInt(m[1], 10);
+        // Plain number (e.g., "30 days", "30")
+        m = t.match(/\b(\d+)\b/);
+        if (m) return parseInt(m[1], 10);
+        return null;
+    }
+
+    function recalcDueDate() {
+        const termsEl = document.getElementById('Terms');
+        const invoiceDateEl = document.getElementById('InvoiceDate');
+        const dueDateEl = document.getElementById('DueDate');
+        if (!termsEl || !invoiceDateEl || !dueDateEl) return;
+
+        const days = parseDays(termsEl.value);
+        if (days === null) return;
+
+        const rawDate = invoiceDateEl.value;
+        if (!rawDate) return;
+
+        // Parse as local date to avoid UTC-offset shifting the day
+        const [y, mo, d] = rawDate.split('-').map(Number);
+        const due = new Date(y, mo - 1, d + days);
+        const yyyy = due.getFullYear();
+        const mm = String(due.getMonth() + 1).padStart(2, '0');
+        const dd = String(due.getDate()).padStart(2, '0');
+        dueDateEl.value = `${yyyy}-${mm}-${dd}`;
+    }
+
+    document.addEventListener('DOMContentLoaded', function () {
+        const termsEl = document.getElementById('Terms');
+        const invoiceDateEl = document.getElementById('InvoiceDate');
+        if (termsEl) {
+            termsEl.addEventListener('change', recalcDueDate);
+            termsEl.addEventListener('blur', recalcDueDate);
+        }
+        if (invoiceDateEl) {
+            invoiceDateEl.addEventListener('change', recalcDueDate);
+        }
+    });
+})();