Presence chip only when others are present

The top-bar presence cluster (avatar stack + "X online" + divider) no longer shows the lonely "just you" state. It appears only once another person joins the space, so a solo session has a cleaner top bar.

Changed

• components/TopBar.tsx — presence block gated on sortedPeople.length > 1 (was > 0); dropped the now-dead "just you" label branch.