The first-run "Invite or add members" step links to /members, but
invite creation only lived in Customize, so the page dead-ended. Reuse
the existing InvitesPanel: the members server page loads invites + space
slug for admin/board and composes the panel into MembersClient via an
inviteSlot prop (MembersClient stays unaware of invites). First-run copy
updated to match.