feature/VSN-2806 - forward isRepresentative flag from frontend for Representative email change flow

The email change flow was fully functional for all User-based entities (driver, client, usager, agent, collaborator) but silently broken for Representatives: the frontend never sent isRepresentative: true to the backend, so both /initiate and /confirm always queried userRepository — which had no record for a Representative UUID — and returned "User not found". This MR wires the flag end-to-end: types → store → the one component that targets a Representative, with zero impact on every other caller.