feat(accounting): VAT-code dropdown in the quote editor (+ reusable VatRateSelect)

Slice 3a — replaces the free-typed VAT rate in the quote editor with a dropdown
of configured output VAT codes (+ 'Other (custom rate)'), reading the un-gated
/admin/vat-codes endpoint. Selecting a code sends vatCode → the backend snapshots
it (migration 130) and the export emits it. New VatRateSelect component + a
read-only vatCodes.service. Create flow snapshots correctly; loading a saved code
into the editor (serialization return) + the bill editor are the next slices.
Build green.

frontend/src/services/quotes.service.ts

@@ -191,6 +191,8 @@ export interface QuoteCreatePayload {
    *  the template's value as-is. */
   installments?: PaymentTermInstallment[];
   vatRate?: number;
+  /** Migration 130 — snapshot of the chosen output VAT code (null = custom rate). */
+  vatCode?: string | null;
   shippingAmountMinor?: number;
   introText?: string;
   outroText?: string;

frontend/src/services/vatCodes.service.ts

@@ -0,0 +1,21 @@
+/**
+ * Read-only VAT-code registry for the invoice/quote editors. Hits the un-gated
+ * /admin/vat-codes endpoint (works without the accounting feature). Management
+ * lives in Settings → Accounting (ledger.service).
+ */
+import { api } from '../config/api';
+
+export interface VatCodeOption {
+  id: number;
+  code: string;
+  name: string;
+  rate: number;
+  direction: 'output' | 'input';
+}
+
+export const vatCodesService = {
+  async listOutput(): Promise<VatCodeOption[]> {
+    const { data } = await api.get('/admin/vat-codes', { params: { direction: 'output' } });
+    return (data?.items ?? []) as VatCodeOption[];
+  },
+};