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.

backend/src/services/invoiceService.js

@@ -1780,6 +1780,8 @@ async function buildInvoiceRenderContext(invoice, lineItems) {
     totals: {
       netAmountMinor: invoice.net_amount_minor,
       vatRate: invoice.vat_rate,
+      // Migration 130 — VAT-code snapshot (so re-editing preserves it).
+      vatCode: invoice.vat_code ?? null,
       vatAmountMinor: invoice.vat_amount_minor,
       shippingAmountMinor: invoice.shipping_amount_minor,
       totalAmountMinor: invoice.total_amount_minor,