Responsive/device-mode missing from preview panel (regression)

The preview panel used to have a built-in responsive/device-size control (drag-to-resize viewport / device presets) directly in the UI, roughly a month ago. It now appears to be gone, so resizing the preview viewport requires asking the model to set widths manually each time.

Expected: a native responsive toggle / draggable viewport in the preview panel, without having to prompt the model.

Actual: no visible responsive control in the preview panel; viewport width can only be changed by asking the model.

Platform: Claude Code desktop app (macOS).