For most users, I want to hide the pose picker if there's not actually anything to pick from.
But I want Support users to be able to open it and use Support mode, to inspect and label Unknown poses!
In this change, I add new UI to show a question mark pose picker button, and a note explaining the empty picker; but only show them for Support users.
I also made a new `useSupport` hook, to replace `useSupportSecret`, now that I have a use case for "is support user?" that doesn't require the secret. Will migrate the rest!
