Explain 4007 user on fleet access page

The [test privileged user](https://github.com/oxidecomputer/omicron/blob/b0d1a3c25dde8ef86469d9f5374b62fdb29c7935/nexus/db-fixed-data/src/silo_user.rs#L12-L25) shows up by default in production systems and is used by support (indirectly — not exactly for authentication or authorization, as far as I know), so it would be inconvenient for it to be deleted. We would like to get rid of it (https://github.com/oxidecomputer/omicron/issues/2305) but in the meantime we can do some harm reduction in the UI to explain what it is. The ID is well-known, so we can just detect it and give it a special name and a tooltip. <img width="1189" height="239" alt="Image" src="https://github.com/user-attachments/assets/e7b26a4f-26c1-4e0a-bae5-e76ac1ed65e5" />