Read --picker-panel from the card's computed style at scene creation
and re-apply on themechange, so the three-js canvases flip with the
rest of the UI instead of staying stuck on the light palette.
Client-side: disable the form submit button until the picker writes a
dataset_id. Server-side: reject posts where dataset_id is present but
empty, instead of silently defaulting to make_s_curve.
