feat: Studio writing UI for case_detail compositions

Stage 6 successor for case_detail composition authoring (parallel to dashboard composer landing in Stage 6).

Blocked on: #507 (unified config-backend ADR) since live-mode writes need the same write-capable backend as dashboard composer.

Scope: live cell preview + section reorder + per-role shell selector + promote modal (post-#507).

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