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.

backend/__tests__/services/expenseService.markup.test.js

@@ -4,7 +4,7 @@
  */
 const expenseService = require('../../src/services/expenseService');
 
-const { computeMarkupMinor, resolveMarkup, computeExpenseAmount, buildExpenseInsert, buildInboundLineItem, isInvoiceMutable } = expenseService._internal;
+const { computeMarkupMinor, resolveMarkup, computeExpenseAmount, buildExpenseInsert, buildInboundLineItem, isInvoiceMutable, resolveTaxTreatment } = expenseService._internal;
 
 describe('computeMarkupMinor', () => {
   it('percent of base, rounded', () => {
@@ -113,6 +113,29 @@ describe('buildInboundLineItem (re-bill line)', () => {
   });
 });
 
+describe('resolveTaxTreatment (supplier-country auto-default)', () => {
+  const reclaim = ['CH', 'LI'];
+  it('explicit valid treatment always wins', () => {
+    expect(resolveTaxTreatment('reverse_charge_service', 'DE', reclaim)).toBe('reverse_charge_service');
+    expect(resolveTaxTreatment('import_goods', 'CH', reclaim)).toBe('import_goods');
+  });
+  it('country in the reclaim list → domestic', () => {
+    expect(resolveTaxTreatment(undefined, 'CH', reclaim)).toBe('domestic');
+    expect(resolveTaxTreatment(null, 'li', reclaim)).toBe('domestic'); // case-insensitive
+  });
+  it('country outside the reclaim list → foreign non-reclaimable', () => {
+    expect(resolveTaxTreatment(undefined, 'DE', reclaim)).toBe('foreign_vat_non_reclaimable');
+    expect(resolveTaxTreatment(undefined, 'US', reclaim)).toBe('foreign_vat_non_reclaimable');
+  });
+  it('unknown / empty country falls back to domestic', () => {
+    expect(resolveTaxTreatment(undefined, '', reclaim)).toBe('domestic');
+    expect(resolveTaxTreatment(undefined, null, reclaim)).toBe('domestic');
+  });
+  it('invalid explicit treatment is ignored (falls through to country logic)', () => {
+    expect(resolveTaxTreatment('bogus', 'DE', reclaim)).toBe('foreign_vat_non_reclaimable');
+  });
+});
+
 describe('isInvoiceMutable (re-categorise unwind guard)', () => {
   const future = new Date(Date.now() + 86400000).toISOString();
   const past = new Date(Date.now() - 86400000).toISOString();