[Feature Request] Exploding messages begin countdown after read (option?)

Exploding messages seem very niche in keybase, because they need to be looked at before they explode. In apps like Signal, the timer for the countdown is different for each recipient, and starts on read, not on send. I think this should be a configurable option the in settings of the chat. Maybe call it countdown on send? Otherwise it makes it hard to use exploding messages.

No files, tests, or entry points are named. Start by tracing exploding-message handling and chat settings across the Go service and React Native client, then compare send-time and read-time behavior. Done means the countdown mode is configurable and supports per-recipient start-on-read timing.

Written by the indexing model from the issue text.