--undo only cleared the reactions log, so a subsequent run kept
diffing forward from each channel's last-seen message instead of
honoring days_back, silently skipping everything already undone.
Undo now also clears channel_scan_state so the next run does a
full fresh backlog scan.
React in 1:1 DMs alongside servers/groups, surface the resolved friend
username at startup and in the stats panel, verify reactions actually
stick after adding them (with jump links for manual spot-checking),
and prune stale targets from stats on each run so only the currently
mutual set is shown.
