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/managers/DragDropManager.ts

@@ -367,7 +367,11 @@ export class DragDropManager {
    * Optimized snap position calculation using PositionUtils
    */
   private calculateSnapPosition(mouseY: number, column: ColumnBounds): number {
-    const snappedY = PositionUtils.getPositionFromCoordinate(mouseY, column);
+    // Calculate where the event top would be (accounting for mouse offset)
+    const eventTopY = mouseY - this.mouseOffset.y;
+    
+    // Snap the event top position, not the mouse position
+    const snappedY = PositionUtils.getPositionFromCoordinate(eventTopY, column);
 
     return Math.max(0, snappedY);
   }