Add a read indicator for web-widget channel

Editor assessment

The subject states the feature concretely, a read indicator for the web-widget channel, and feat is right, but no body explains the indicator's behavior, when it appears for the visitor versus the agent, which is the UX contract the feature exists to define. No scope, no footers beyond Co-authored-by. The trailing pull-request number is the conventional squash-merge marker. Hygiene is good: imperative shape, no trailing period, no junk, moderate length. Clear headline, absent behavior detail, the standard acceptable profile. Who sees the indicator and when is unstated.