Ports the gooyal notifications client (as already used by advertising)
and wires it into the previously-empty PushPublisher: on each message
send, the other conversation participant(s) get a push notification
with the sender's name and a message preview. Failures are swallowed
per-recipient so a down notifications service degrades silently rather
than breaking message sending.
Requires NOTIFICATIONS_BASE_PUBLIC_URL and the
notifications.application.push:send OAuth2 scope to be configured
before it will actually deliver.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
