Surface roster validation failures when meeting start is rejected

Stress Chrome testing with large multi-list rosters prepared meetings entirely through Oppmøteoversikt. Starting returned HTTP 422 and left the start control visible, but the modal closed without any error. Voting-engine receipts recorded `holder_unavailable` and `replacement_attendance_mismatch`. A second controlled run on fresh Stress Dynamo reproduced `replacement_attendance_mismatch` six times after the UI showed Forfall 4/4 and Delvis 4/4, including after changing adjacent shared-person assignments back to distinct deputies; `VotingEngine::Roster#initial_paths` still rejects the prepared state. A secretary must get a visible, actionable explanation identifying the conflicting attendance/replacement so the roster can be repaired. Also investigate why supported availability/replacement changes leave attendance state inconsistent with accepted invitations. Keep startup atomic and add coverage for invalid accepted full/partial relationships and unavailable holders.