Plan 3A: backup & restore — drives the feature branch landing in
the next commit (merge of feature/backup-restore).
Pre-v0.3.0 audit checklist: manual smoke-test list for the v0.2.x
audit-pass commits (TOTP edit, history, detach, status, generator
defaults, vault-tab parity, sync button) — to walk through before
the v0.3.0 tag.
