A Slack-like "fake Slack" dashboard, rendered live. A host app can mount it
with live "/dev/slack", Slackbox.DashboardLive.
It subscribes to the Slackbox.Store PubSub topic, so messages stored by the
Slackbox.Adapters.Local adapter appear in real time. Each message renders
its Block Kit blocks and offers a per-message "raw payload" toggle.