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).

frontend/src/components/admin/VatRateSelect.tsx

@@ -38,10 +38,15 @@ export const VatRateSelect: React.FC<Props> = ({ rate, code, onChange, label, di
   });
 
   // Selected option: prefer the snapshotted code; else a code whose rate matches
-  // (legacy rows / no code stored); else the document's value is "off-list".
+  // (legacy rows / no code stored) — BUT only when that rate is unambiguous. If
+  // two configured codes share the rate (e.g. two 8.1% codes), rate-matching
+  // could silently swap one for the other on the next save, so fall through to
+  // the legacy "(not configured)" option and make the admin pick explicitly
+  // (PR #636 review #4).
   const matched: VatCodeOption | undefined =
     (code ? codes.find((c) => c.code === code) : undefined)
-    || (!code ? codes.find((c) => Number(c.rate) === Number(rate)) : undefined);
+    || (!code && codes.filter((c) => Number(c.rate) === Number(rate)).length === 1
+      ? codes.find((c) => Number(c.rate) === Number(rate)) : undefined);
   const showLegacy = !matched;
 
   return (
@@ -61,7 +66,7 @@ export const VatRateSelect: React.FC<Props> = ({ rate, code, onChange, label, di
       >
         {showLegacy && (
           <option value={LEGACY}>
-            {t('vat.legacyRate', '{{rate}}% (not configured)', { rate: Number(rate || 0).toFixed(1) })}
+            {t('ledger.vat.legacyRate', '{{rate}}% (not configured)', { rate: Number(rate || 0).toFixed(1) })}
           </option>
         )}
         {codes.map((c) => (