Add an admin toggle for superusers

It is often very useful to test out functionality in the deployed DANDI instance from the perspective of a non-superuser. I'd like to add a toggle to the UI somewhere, only visible to superusers, that can be used to disable superuser powers at will. It would work like this:
- the switch appears for all superusers, no matter what
- if the switch is on, then the app behaves as it does right now
- if the switch is off, then the app behaves as though you are _not_ a superuser (with the exception that it still shows the switch)
- the switch should be off by default

This would obviate the need to log in as dandibot in a private window, etc.

The issue names no files, tests, or entry points. Start by locating the superuser checks and the main UI navigation or account controls, then trace how authentication state reaches the API and frontend. Done means every superuser sees a default-off toggle, and disabling it removes superuser behavior while leaving the toggle usable.

Written by the indexing model from the issue text.