Standalone maintenance page LiveView for admin preview and direct access.
The main maintenance enforcement is handled by the layout override in
check_maintenance_mode/1 (auth.ex) — non-admin users see the maintenance
layout on whatever page they're on, without a URL change.
This LiveView serves as:
- Admin preview at
/maintenance(with banner + settings link) - Fallback target for the plug's 503 response on non-LiveView controller routes
Supports countdown timer, PubSub status updates, and admin preview.