Select the "main" slide for the shared whiteboard (MED-976 / E3.3a)

Sets current_slide_page on the session — the slide-deck page rendered large on the live whiteboard. The page must be one of the CURRENT section's associated slides. Persisted on the session and reflected in both the coach and participant active-state payloads, so every attendee sees the same slide ("shared with all"). Pass null to clear the selection (falls back to the section's first associated slide).