backup: the versioned .waybetter envelope (format/version checks, only the
app's registered sections kept), secret-key scrubbing, the plugins /
customCss / accounts sections every app has, plugin import checks and
atomic writes. sync: Argon2id + XChaCha20-Poly1305 snapshots of the
settings backup in a folder the owner syncs, moved unchanged from the
wayBetter for X app with its tests.
