feat(admin): transaction management

Sprint 24.

New features/admin/transactions/ module. AdminTransactionsLocalGateway
derives one synthetic transaction per Sprint 23's seeded mock order rather
than a separate dataset, keeping order numbers/totals consistent across
the two mock feature areas.

- list: search, status filter, type filter (payment/refund/qr_payment),
  pagination, CSV export
- retry failed transactions (appends an audit entry)
- fraud flag toggle
- per-transaction audit log (creation/retry/fraud-flag-change), viewed via
  dialog - intentionally separate from the system-wide audit/security log
  planned for Sprint 26 (Monitoring)
- wired into /:lang/backoffice/transactions, replacing the coming-soon
  placeholder

docs/ADMIN.md + docs/BACKEND.md (new item 13) updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/SPRINT-PLAN.md

@@ -47,8 +47,9 @@ Notify user: **from Sprint 20 (Categories) once product↔category link + admin
 - [x] `docs/ADMIN.md` (new Sprint 23 section), `docs/BACKEND.md` item 7 rewritten
 - Commit: `feat(admin): order management`
 
-## Sprint 24 — Transactions (mock/local)
-- [ ] Payments/refunds/QR list, status, history, export, filters, search, retry, fraud flags, audit log view
+## Sprint 24 — Transactions (mock/local) ✅ done
+- [x] Payments/refunds/QR list, status, history (derived from Sprint 23's seeded orders), export, filters, search, retry failed, fraud flags, per-transaction audit log dialog
+- [x] `docs/ADMIN.md` (new Sprint 24 section), `docs/BACKEND.md` item 13 added
 - Commit: `feat(admin): transaction management`
 
 ## Sprint 25 — Users & Roles