feat(mail-overview): notify per account when mail arrives

Asks notmuch what changed since the revision last seen, then sends
one notification per account with mail in that window. Counts are
validated as integers because a rejected notmuch query prints nothing
and exits 1, and an empty string must not read as zero.

dunstify is backgrounded because -b blocks until the notification is
dismissed, which would otherwise stall the loop for the full timeout
on every account. The watch is on the xapian directory rather than a
file inside it, because a commit replaces files and a watch held on a
filename dies with it.

0 files changed, 0 insertions, 0 deletions