Get participant dashboard data

Returns all data needed for the participant dashboard, scoped to the authenticated participant. Includes cohort info, announced modalities with progress, upcoming sessions, starting soon banner data (sessions within 30 minutes), and action items (unscheduled 1:1 sessions). When the live_session_delivery feature flag is on, a non-null refresh object is included carrying the MED-938 / E3.5 two-column dashboard data (unified upcoming list with peer-leader state, completed sessions with feedback status, program/cohort-manager/group sidebar data, modality progress, and cohort-level resources). When the flag is off, refresh is null and the legacy fields drive the dashboard.