Add a dedicated /api/secrets/export route (api/secrets/export.js)
that streams the unified backup format (JSON/CSV/HTML/TXT) with size
and auth guards. Settings persist the preferred export format per
user; backup runs pick it up so generated files match the chosen
schema. The UI settings label/toast reads "导出偏好格式" to clarify
the preference applies to batch export and backup export. The
Service Worker passes through offline responses, utils/backup-format.js
is slimmed down after route consolidation, and unused createdAt is
dropped from the unified format and validation.
