Add the shared SessionForm component (create/edit) used by the Sessies screen.
- Create mode: worker picker (useAdminUsers), activity (useActivities), zooltype,
pair count, start/end datetime-local, pauze, notitie; posts CreateManualSessionInput.
- Edit mode: hides the worker picker, prefills from the row session, adds a status
select; PUTs AdminUpdateSessionInput (end_time nulled when status=active).
- Builds ISO start_time/end_time from the datetime-local values; live "gewerkt"
preview = end - start - paused; inline error on a 400.
- Wires the form into Sessions.tsx ("+ Nieuwe registratie" -> create, pencil -> edit).
Products affected: SoleLog admin client
69b46bee04