Proxy- and hook-authenticated users were auto-provisioned by applying the
default scope (".") and passing it straight to MakeUserDir, which normalizes
"." to "/". With CreateUserDir enabled, every provisioned user therefore
received the server root as its scope instead of a per-user home directory,
letting one user read, overwrite and delete another user's files.
The signup handler already cleared the scope before deriving the home
directory. Centralize that logic into Settings.CreateUserHome (clear the
scope when CreateUserDir is on and no explicit scope was supplied, derive the
home dir, then reject a scope already owned by another user) and use it from
signup, proxy and hook auth so the three provisioning paths cannot diverge.
Refs GHSA-j7jh-37pf-mf8h, GHSA-j2fc-28fx-hc8q
