notification.py now handles all three outbound channels. Email defaults
to the user's login address (google_email from auth.json); an optional
override can be set in channels.json. Google Chat uses an incoming
webhook URL. NC Talk was already wired, just needs notification_room set.
Settings page gains a Notifications section: channel dropdown, optional
email override, NC room token, and Google Chat webhook URL. All stored
in per-user channels.json.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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