The switcher only offered 'sign out of active' and 'sign out of all' — no
way to drop a single non-active account (e.g. one stuck in an error state you
can't switch into to sign out). Add a hover × on non-active, non-default rows
and a removeAccount(id) auth action that tears down the client, drops it from
the registry, and clears its per-slot session/token cookies.
Stacks on the switcher redesign in #517.
