Gantt / waterfall view of a project — the same data as
ProjectShowLive, rendered as horizontal bars on a date axis via the
PhoenixLiveGantt component instead of the vertical timeline.
Read-only: it visualizes the project's assignments as a sequential
schedule (each task starts where the previous one ends, honoring the
project's weekday/weekend rules via Project.eta_from/3) and draws
dependency arrows between them. Mutations still happen on the vertical
show page; the two views share the project UUID.