Regression: SortZone calculates item layout after placeholder insertion, causing incorrect positioning

When dragging items in a SortZone where items have margins (e.g., &:not(:first-child) { margin-left: 20px; }), the layout calculations (getDomRect) were previously occurring after dragStart.

dragStart inserts a dragPlaceholder which modifies the DOM structure. If this placeholder doesn't perfectly mimic the original item's margins immediately, the remaining siblings shift positions (e.g. collapsing the gap). Since getDomRect was called after this shift, the captured itemRects were incorrect (missing the gaps), causing items to overlap when absolute positioning was applied.

Fix: The onDragStart logic in src/draggable/container/SortZone.mjs has been reordered to:

1. Measure the DOM (getDomRect) and capture itemStyles before calling dragStart.
2. This ensures the layout is captured exactly as rendered, with all margins and gaps intact.
3. Apply these correct coordinates after the drag operation has started.