The no-banner placeholder was a short h-14 (56px) bar while a real banner renders
~150px, so the -mt-8 avatar rode up into the absolutely-positioned Back button on
banner-less channels (e.g. Billet Box). Give the fallback the same aspect-ratio +
maxHeight as the banner container so the overlapping avatar has the same room and
the header layout is consistent whether or not a channel has a banner.