`LoadSettingsForInstall` only ran `loadMailerFrom`, not
_loadRegisterMailFrom_ or _loadNotifyMailFrom_, so
Service.RegisterEmailConfirm and Service.EnableNotifyMail were never
read from app.ini on the install page.
Full startup runs those through loadMailsFrom; the install path was a
narrower subset and never included that step—an oversight from when
install-specific loading was added
Fixes#37112