app: kill rsync when a sync is stopped

A stopped worker (schedule replaced, constraints lost) left rsync
running detached until its own network timeout, including a window
where a WiFi-only sync kept pushing on another network. A watchdog
coroutine destroys the process on cancellation, which also closes its
pipes and unblocks the log reader; the worker rethrows cancellation
instead of logging it as a folder error.

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