Explain active-poll conflicts when participant changes are blocked

In meeting 16, changing the active replacement from Klara Ku to Ole Brumm during an active poll was correctly rejected with HTTP 422 and no attendance or canonical mutation. The UI incorrectly said `Varaen er ikke lenger tilgjengelig. Listen er oppdatert.` Ole remained eligible and the exact transition succeeded immediately after poll completion. Surface the voting engine's active-poll conflict with actionable guidance to finish or cancel the poll before changing participants, and preserve the existing atomic rollback. Cover replacement, absence, restoration, and fill-seat actions while a poll is active.