The default on_mount hook and live_session session builder for the
SquatchMail dashboard.
Access control is fully handled ahead of live_session by
SquatchMail.Web.Plugs.Auth (see SquatchMail.Web.Router for why it can't
live here); this module is strictly post-auth. It assigns the dashboard's
base path so layout components (nav links, asset URLs) can build paths
relative to wherever the host mounted squatch_mail_dashboard. It is
always appended last to the on_mount list so host hooks configured via
squatch_mail_dashboard(path, on_mount: [...]) run first.