feat(mail-overview): show mail-watcher health in the drawer

The watcher already writes ~/.local/state/mail-watcher.heartbeat every
60s, so the drawer reads it and shows a dot: green when idling, yellow
when a folder gave up, red when the heartbeat is stale or missing.

Backoff stays green because it is normal recovery and the watcher's own
health check treats it that way. The staleness rule is reimplemented in
a few lines rather than shelling out to --status; the heartbeat file is
the documented cheap-to-poll surface.

Read once per open rather than watched: the heartbeat is written by
atomic replace, so an inotify watch held on the old inode dies with it.

"Sync now" is removed with its Process. The watcher triggers syncs, the
cron tick is the backstop, and waybar's right-click still runs
mailsync.sh.

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