fix(accounting): address the-luap PR #636 review

- #1 resolveTaxTreatment: an unconfigured (empty) reclaim-countries list no
  longer auto-classifies every supplier — incl. the admin's own domestic one —
  as foreign; defer auto-classification until the setting is set (+ test).
- #2 pending re-bills on customer erase: eraseCustomer now returns the
  customer's not-yet-billed inbound docs to the inbox (null customer + unsorted)
  so they aren't billable to an anonymized account. (NB: picpeak has no hard
  customer delete — erase anonymizes in place — so the orphan/404 premise can't
  occur; this is hardening.)
- #4 VatRateSelect: when >1 configured code shares the same rate, fall through
  to the legacy "(not configured)" option instead of silently picking the first.
- #5 unwindBilledLine: delete the (mutable, never-issued) invoice when the
  unwound re-bill was its only line, instead of leaving a net-zero survivor.
- #6 isInvoiceMutable: clarify in a comment that invoices have no 'draft' status
  (the editable state is 'scheduled' w/o send-at) — no behaviour change.
- nit: collapse normalizeCurrency's tautological ternary.
- Fix VAT picker i18n: t('vat.legacyRate') → 'ledger.vat.legacyRate' (the key's
  real home), so the legacy label localizes instead of always showing English.
- Remove dead i18n keys left by the settings refactor (businessProfile.field VAT
  /hourly + profileFields.title/savedToast).

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

@@ -131,6 +131,11 @@ describe('resolveTaxTreatment (supplier-country auto-default)', () => {
     expect(resolveTaxTreatment(undefined, '', reclaim)).toBe('domestic');
     expect(resolveTaxTreatment(undefined, null, reclaim)).toBe('domestic');
   });
+  it('an UNCONFIGURED (empty) reclaim list never auto-classifies as foreign (PR #636 #1)', () => {
+    expect(resolveTaxTreatment(undefined, 'CH', [])).toBe('domestic');
+    expect(resolveTaxTreatment(undefined, 'DE', [])).toBe('domestic');
+    expect(resolveTaxTreatment(undefined, 'US', undefined)).toBe('domestic');
+  });
   it('invalid explicit treatment is ignored (falls through to country logic)', () => {
     expect(resolveTaxTreatment('bogus', 'DE', reclaim)).toBe('foreign_vat_non_reclaimable');
   });