Postr

Postr does one job: compose a kind-1 note, sign it with Amber, and send it
to your relays.

Its durable outbox survives airplane mode, process death, and unreliable
relays. Postr verifies the exact signed event by reading it back, and every
retry reuses that event so it cannot create a duplicate note.

Private keys stay in Amber. Postr asks only to sign kind-1 notes and
file-scoped Blossom upload authorizations. It has no feed, analytics, ads,
or private-key storage.