Feat(Active Round, Stage 0): Card to show registration status

For admins and participants show waiting message while there are open spots in the round
For admins, show ready message and button to start the round

The issue does not name files, tests, or entry points. Start by locating the active-round registration UI and identify where participant and admin states are rendered. Done means participants and admins see a waiting message while spots remain, while admins see a ready message and a control to start the round.

Written by the indexing model from the issue text.