feat: live cell previews in case_detail Studio composer

Studio composer for case_detail needs live cell previews so jurisdiction admins can see what each section will look like in real time as they reorder/configure.

Blocked on: #563 (case_detail Studio writing UI) — this FU is the preview leg.

Scope: iframe or in-page preview surface; per-shell preview (tabs/scroll/card_grid); axe-AA preserved during preview.

Tracking origin: Stage 5 MR4 (#497 (closed)) descope. Refs &51.