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/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