feat(accounting): bill editor VAT dropdown + GET returns vat_code snapshot

Slice 2 + 1b:
- Bill editor: VAT-rate field → VatRateSelect dropdown (mirrors the quote
  editor); snapshots vatCode on create + carries it from a source quote.
- getQuoteById + the invoice serializer now return vat_code, so re-editing a
  saved document preserves the snapshot instead of falling back to the
  rate→code map. Payload types (quotes + bills) carry vatCode.

72 tests pass; build green.

frontend/src/pages/admin/bills/BillEditorPage.tsx

@@ -14,6 +14,7 @@ import { quotesService } from '../../../services/quotes.service';
 import { contractsService } from '../../../services/contracts.service';
 import { businessProfileService } from '../../../services/businessProfile.service';
 import { CustomerPicker } from '../../../components/admin/CustomerPicker';
+import { VatRateSelect } from '../../../components/admin/VatRateSelect';
 import { LineItemsTable, type EditableLineItem } from '../../../components/admin/LineItemsTable';
 import { InstallmentsPanel } from '../../../components/admin/InstallmentsPanel';
 import { customerAdminService } from '../../../services/customerAdmin.service';
@@ -58,6 +59,8 @@ export const BillEditorPage: React.FC = () => {
   // effect; new invoices start as null so they pick up profile.)
   const [qrFormat, setQrFormat] = useState<InvoiceQrFormat | null>(null);
   const [vatRate, setVatRate] = useState(0);
+  // Migration 130 — snapshot of the chosen output VAT code (null = custom rate).
+  const [vatCode, setVatCode] = useState<string | null>(null);
   const [shipping, setShipping] = useState(0);
   const [ccPdfEmail, setCcPdfEmail] = useState('');
   const [lineItems, setLineItems] = useState<EditableLineItem[]>([]);
@@ -137,6 +140,8 @@ export const BillEditorPage: React.FC = () => {
       // it inherits the profile default at render time.
       setQrFormat((inv.qrFormat as InvoiceQrFormat | null) || null);
       setVatRate(Number(inv.vatRate || 0));
+      setVatCode(((inv as { vatCode?: string | null }).vatCode
+        ?? (inv as { totals?: { vatCode?: string | null } }).totals?.vatCode) ?? null);
       setShipping(Number(inv.shippingAmountMinor || 0) / 100);
       setCcPdfEmail(inv.ccPdfEmail || '');
       setPaymentTermTemplateId(inv.paymentTermTemplateId ?? null);
@@ -280,6 +285,7 @@ export const BillEditorPage: React.FC = () => {
           const { quote, lineItems: quoteLineItems } = await quotesService.get(contract.sourceQuoteId);
           if (quote.currency) setCurrency(quote.currency);
           if (quote.vatRate != null) setVatRate(Number(quote.vatRate));
+          setVatCode((quote as { vatCode?: string | null }).vatCode ?? null);
           if (quote.shippingAmountMinor != null) {
             setShipping(Number(quote.shippingAmountMinor) / 100);
           }
@@ -373,6 +379,7 @@ export const BillEditorPage: React.FC = () => {
     // which is the same outcome but pollutes the column.
     qrFormat: qrFormat || undefined,
     vatRate,
+    vatCode,
     shippingAmountMinor: toMinor(shipping),
     ccPdfEmail: ccPdfEmail || undefined,
     // Payment-term template id (migration 113). null = no template
@@ -651,8 +658,11 @@ export const BillEditorPage: React.FC = () => {
         <LineItemsTable items={lineItems} currency={currency} showDiscount={false}
           vatRate={vatRate / 100} shippingAmount={shipping} onChange={setLineItems} />
         <div className="grid grid-cols-1 md:grid-cols-3 gap-3 mt-4">
-          <Input type="number" step="0.1" label={t('bills.field.vatRate', 'VAT rate %') as string}
-            value={vatRate} onChange={(e) => setVatRate(Number(e.target.value))} />
+          <VatRateSelect
+            label={t('bills.field.vatRate', 'VAT rate %') as string}
+            rate={vatRate}
+            code={vatCode}
+            onChange={(rate, code) => { setVatRate(rate); setVatCode(code); }} />
           <Input type="number" step="0.01" label={t('bills.field.shipping', 'Shipping') as string}
             value={shipping} onChange={(e) => setShipping(Number(e.target.value))} />
           <div>

frontend/src/services/bills.service.ts

@@ -180,6 +180,8 @@ export interface InvoiceCreatePayload {
   installmentLabel?: string;
   installmentTrigger?: string;
   vatRate?: number;
+  /** Migration 130 — snapshot of the chosen output VAT code (null = custom rate). */
+  vatCode?: string | null;
   shippingAmountMinor?: number;
   ccPdfEmail?: string;
   // null = explicitly clear the per-invoice override (back to the