cinny-threads: refresh thread unread badge on read receipts
The thread unread badge in RoomViewHeader only recomputed on
ThreadEvent.New/Update/NewReply. Marking a thread read emits
RoomEvent.Receipt / RoomEvent.UnreadNotifications instead, so the badge
kept a stale count until unrelated thread activity fired a recompute -
the notification appeared persistent and laggy. Subscribe the badge
effect to those receipt events too.
Assisted-by: Claude Code (claude-opus-4-8[1m])