Handle participant overview before voting run initialization

Chrome acceptance reproduced an HTTP 500 on fresh not-started meetings 6 and 14. Opening /meetings/:id/secretary/participants before meeting start raises VotingEngine::Conflict run_not_found from app/services/voting_engine/participant_overview.rb:41 because Query.current has no initialized run. All other pre-start meeting tabs render normally. Render an explicit pre-start participant state (or attendance-based preparation view) instead of raising, while preserving canonical-only current voting state once a run exists. Cover direct requests, Turbo tab navigation, and hard refresh.