Scale Tree.DRAG_IMAGE_SIZE by zoom level instead of using fixed pixels

The Tree.DRAG_IMAGE_SIZE constant specifies the width of the drag image when a tree item is dragged. Previously, this value was fixed in pixels, which caused the drag image to appear too small on high-DPI monitors.

This change redefines DRAG_IMAGE_SIZE in points and converts it to pixels based on the current zoom level, ensuring consistent drag image sizing across different display scales.