feat(theme-studio): compact the box control into a 2x2 button cluster

The box control was a wide select plus a color swatch, pinning the box column at 166px. I replaced the select with a 2x2 cluster of radio buttons for the four styles: blank (no box), □ (line), ▼ (pressed), ▲ (raised). The color swatch now shows only while a box style is active, so the no-box case stays narrow. The column drops to 76px across all three tiers.

A #boxtest gate covers the cluster: four buttons, radio selection, and the swatch hiding when no box is set. #beveltest now drives the style through the cluster button instead of the removed select.

The same cluster shape sets up the B/I/U/S style column next.

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