feat(accounting): supplier-country tax default + configurable default output VAT code

VAT supplier-country reclaim default:
- Migration 134 adds inbound_documents.supplier_country.
- categorizeInbound auto-derives tax_treatment via resolveTaxTreatment:
  explicit treatment wins; else country in the reclaim list → domestic,
  outside it → foreign_vat_non_reclaimable, unknown → domestic. Consumes the
  previously-stored-but-unused accounting_vat_reclaim_countries.
- Triage modal gains a Supplier country dropdown (saved via updateInbound).
  +5 unit tests for resolveTaxTreatment.

Configurable default output VAT code for new invoices:
- New accounting_default_output_vat_code setting (PUT wired; getSettings/type).
- Settings → Accounting dropdown to pick it.
- Invoice + quote editors seed their VAT picker (rate + code) from it on a
  blank new document — skipping edits/conversions, never clobbering a touched
  value. New docs no longer silently start at 0%.

i18n en + de.

frontend/src/services/accounting.service.ts

@@ -39,6 +39,8 @@ export interface InboundDocument {
   customerAccountId: number | null;
   customerName: string | null;
   customerEmail: string | null;
+  /** ISO-2 supplier country — auto-defaults the tax treatment (reclaim list). */
+  supplierCountry: string | null;
   /** Free-text categorisation note. */
   note: string | null;
   supplierPaid: boolean;
@@ -112,6 +114,8 @@ export interface AccountingSettings {
   /** ISO-2 countries whose input VAT can be reclaimed (drives cost
    *  tax-treatment + the report's VAT-payable). */
   accounting_vat_reclaim_countries: string[];
+  /** Output VAT code stamped onto NEW invoices/quotes ('' = none). */
+  accounting_default_output_vat_code: string;
 }
 
 export interface CategorizePayload {
@@ -210,6 +214,8 @@ export const accountingService = {
       accounting_vat_registered: data.accounting_vat_registered === true,
       accounting_vat_reclaim_countries: Array.isArray(data.accounting_vat_reclaim_countries)
         ? data.accounting_vat_reclaim_countries : [],
+      accounting_default_output_vat_code: typeof data.accounting_default_output_vat_code === 'string'
+        ? data.accounting_default_output_vat_code : '',
     };
   },
   async updateSettings(payload: Partial<AccountingSettings>): Promise<{ updated: string[] }> {