Administration's pages are tables, and the Settings-style second column
took width they need. The list of sections -- Directory > Accounts,
Mail > Domains -- now sits in the folder pane where Mail keeps its folders,
and the page is the open section alone, up to 1120px wide.
On a phone the list is in the drawer like every other section's, so a bare
/admin opens the first section rather than a page that is only a list. The
back link it needed is gone.