fix(ui): logout type error when user is null during locale switch (#13514)

### What?

Prevent a `TypeError: Cannot read properties of null (reading
'collection')` in the admin UI when switching locales by hardening
`AuthProvider.logOut`.

### Why?

During locale transitions, user can briefly be null. The existing code
used `user.collection` unguarded

### How?

- Use `userSlug` over `user.collection`.
- Always clear local auth in a `finally` block (`setNewUser(null)`,
`revokeTokenAndExpire()`), regardless of request outcome.

Fixes #13313 


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