Deliverability alerts from MailChannels now surface in Reach

When MailChannels flags a sending issue, Reach surfaces it as a banner across main pages. A persistent banner appears for blocked domains or sender emails (due to spam complaints or abuse signals), directing users to review their content and practices before contacting support. A dismissible warning alerts users to poor deliverability with a note that sending may be suspended if it doesn't improve. This brings critical delivery signals directly into the user's workflow so they can act before problems escalate.