feat(backoffice): family email from driver planning with multi-passenger recipients

Add the family-email planning drawer wired from the chauffeur schedule: load trip passengers, multi-select users, choose representatives per user in one flat list (with passenger context), message templates and subject/body, and send a single batch request to the user-service planning family-email endpoint. Update API types and client payload for passengers[]. Refine PassengerCard (multi-select) and RepresentativeRow (stable ids, optional passenger label). Remove the duplicate per-passenger recap block from the recipients UI.