feat(theme-studio): expand/collapse-all toggle and disclosure triangles

Each row's expander toggle now shows a disclosure triangle that tracks its state: a right triangle when collapsed, a down triangle when expanded (it was a static ellipsis). A header-level expand-all / collapse-all button per table opens or closes every row's detail at once and follows the aggregate state. The per-row triangles and the header button stay in sync across a table rebuild.

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