fix: remove calcDragPosition from default strategies to fix drag position jump (#2223)

The default transformStrategy and absoluteStrategy were returning screen
coordinates (clientX - offsetX) in calcDragPosition, but the drag system
expects parent-relative coordinates. This caused items to jump down by the
distance from the top of the page when dragging started.

The fix removes calcDragPosition from these default strategies, allowing
react-draggable to use its built-in parent-relative coordinate calculation.
Only createScaledStrategy retains calcDragPosition since it needs to adjust
for the scale factor.

Also makes calcDragPosition optional in the PositionStrategy interface, as
it's only needed for custom coordinate handling (e.g., scaled containers).

Bug was introduced in commit 0c80a9f which added the PositionStrategy
extension point with default calcDragPosition implementations.

Includes regression test to verify items don't jump during drag.