slab: mail (mbsync + mu) integration in menubar
Adds a "Mail" menu item showing unread counts across ac-mail and jas-mail
maildirs. Includes inline actions to sync either/both channels in the
background and open the sync log. Polls mu sparingly (every ~30s) unless a
mbsync is in flight, in which case the status string shows a "syncing…"
suffix.
Also gitignores .claude/scheduled_tasks.lock so the runtime lock file
doesn't show up as untracked noise.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>