feat(mail-overview): waybar watcher in continuous mode

Prints one JSON line per notmuch commit rather than polling on an interval,
so the count drops the moment mail is read in qtmaildir and rises the moment
mbsync commits, and waybar owns the watcher process: nothing to supervise on
a machine with no systemd.

The watch is on the xapian directory, not on a file inside it, because a
commit replaces files and a watch held on a filename dies with it. The short
sleep coalesces the several writes of one commit into one redraw.

notmuch exits 0 even for a malformed query, printing something that is not a
count, so the output is validated as an integer rather than trusting the exit
status. A failure there renders as an error glyph: a count that silently
reads zero would look exactly like an empty inbox.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WWL8JYHu7yhAdtx5pU9PMU

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