Improves all-day event drag and drop

Addresses issues with all-day event duration calculation and positioning during drag and drop.

- Uses `differenceInCalendarDays` to correctly calculate event duration across timezone and DST boundaries.
- Preserves the original event time when moving events to a different day.
- Snaps dragged event to the top of the target time slot.

src/renderers/EventRenderer.ts

@@ -239,8 +239,9 @@ export class DateEventRenderer implements EventRendererStrategy {
   public handleDragMove(payload: DragMoveEventPayload): void {
     if (!this.draggedClone) return;
 
-    // Update position
-    this.draggedClone.style.top = (payload.snappedY - payload.mouseOffset.y) + 'px';
+    // Update position - snappedY is already the event top position
+    // Add +1px to match the initial positioning offset from SwpEventElement
+    this.draggedClone.style.top = (payload.snappedY + 1) + 'px';
 
     // Update timestamp display
     this.updateCloneTimestamp(payload);