* Make preview slider labels draggable
Add label hit testing and delta-based dragging for the vertical preview slider labels. Keep label drags tied to the selected handle, prevent slider hover/timeline/menu handling from stealing label interactions, and keep value setters from changing the active selection implicitly.
* Refresh preview slider visuals
Update preview slider rails, handles, and labels for the refreshed light and dark theme appearance. Apply the same visual language to the horizontal slider, align single-layer and multi-layer labels, and remove obsolete triangle label geometry.
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