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/migrations/core/134_add_inbound_supplier_country.js

@@ -0,0 +1,25 @@
+/**
+ * Migration 134: supplier country on incoming invoices.
+ *
+ * `supplier_country` (ISO-3166 alpha-2) lets categorisation auto-default the
+ * `tax_treatment`: a supplier whose country is in the install's VAT reclaim
+ * list (Settings → Accounting → `accounting_vat_reclaim_countries`, typically
+ * CH / LI) → `domestic` (input VAT reclaimable); otherwise →
+ * `foreign_vat_non_reclaimable`. Closes the dangling VAT-consolidation slice
+ * where the reclaim-countries setting was stored but never consumed.
+ *
+ * Additive + hasColumn-guarded.
+ */
+exports.up = async function (knex) {
+  if (!(await knex.schema.hasTable('inbound_documents'))) return;
+  if (!(await knex.schema.hasColumn('inbound_documents', 'supplier_country'))) {
+    await knex.schema.alterTable('inbound_documents', (t) => t.string('supplier_country', 2));
+  }
+};
+
+exports.down = async function (knex) {
+  if (!(await knex.schema.hasTable('inbound_documents'))) return;
+  if (await knex.schema.hasColumn('inbound_documents', 'supplier_country')) {
+    await knex.schema.alterTable('inbound_documents', (t) => t.dropColumn('supplier_country'));
+  }
+};