Persist config, key blob, and host pin via AtomicFile so an
interrupted write cannot corrupt state. Validate the port, clear the
host pin when the remote changes, cap imported key size, time out
native helpers, and escalate rsync termination to destroyForcibly.
Serialize manual and scheduled syncs behind a mutex.

Replace the chained one-time jobs with plain periodic work: it
survives reboots without a boot receiver and cannot silently die
like a broken chain. Costs the sub-15-minute interval, which photo
backup does not need. Document the floor and that syncs run on any
unmetered network, not only WiFi.

Typing absolute paths is error-prone; let the user browse real
filesystem folders and pick one. Stays on real paths (no SAF) so
rsync keeps operating on POSIX paths.

A small Android app for one-way folder backup, a KISS alternative to
Syncthing. It bundles rsync (built from pinned source via the NDK) and
a pure-Go SSH transport, both shipped in the APK as lib*.so and run from
the native library directory.

rsend pins the host key, stores the ed25519 identity Keystore-encrypted,
pushes each folder additively or as a mirror, and runs on demand or on a
WiFi-only schedule. The build is self-contained and reproducible: make
setup provisions the toolchain, make builds rsync, rsh, and the APK.