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/AllDayManager.ts

@@ -17,6 +17,7 @@ import {
 import { DragOffset, MousePosition } from '../types/DragDropTypes';
 import { CoreEvents } from '../constants/CoreEvents';
 import { EventManager } from './EventManager';
+import { differenceInCalendarDays } from 'date-fns';
 
 /**
  * AllDayManager - Handles all-day row height animations and management
@@ -379,7 +380,9 @@ export class AllDayManager {
         throw new Error('Ugyldig start eller slut-dato i dataset');
       }
 
-      return Math.round((endDate.getTime() - startDate.getTime()) / (1000 * 60 * 60 * 24));
+      // Use differenceInCalendarDays for proper calendar day calculation
+      // This correctly handles timezone differences and DST changes
+      return differenceInCalendarDays(endDate, startDate);
     };
 
     if (dragEndEvent.draggedClone == null)
@@ -403,10 +406,23 @@ export class AllDayManager {
 
 
     const durationDays = getEventDurationDays(dragEndEvent.draggedClone.dataset.start, dragEndEvent.draggedClone.dataset.end);
-    const newStartDate = new Date(eventDate);
-    const newEndDate = new Date(newStartDate);
-    newEndDate.setDate(newEndDate.getDate() + durationDays);
     
+    // Get original dates to preserve time
+    const originalStartDate = new Date(dragEndEvent.draggedClone.dataset.start!);
+    const originalEndDate = new Date(dragEndEvent.draggedClone.dataset.end!);
+    
+    // Create new start date with the new day but preserve original time
+    const newStartDate = new Date(eventDate);
+    newStartDate.setHours(originalStartDate.getHours(), originalStartDate.getMinutes(), originalStartDate.getSeconds(), originalStartDate.getMilliseconds());
+    
+    // Create new end date with the new day + duration, preserving original end time
+    const newEndDate = new Date(eventDate);
+    newEndDate.setDate(newEndDate.getDate() + durationDays);
+    newEndDate.setHours(originalEndDate.getHours(), originalEndDate.getMinutes(), originalEndDate.getSeconds(), originalEndDate.getMilliseconds());
+
+    // Update data attributes with new dates
+    dragEndEvent.draggedClone.dataset.start = newStartDate.toISOString();
+    dragEndEvent.draggedClone.dataset.end = newEndDate.toISOString();
 
     const droppedEvent: CalendarEvent = {
       id: eventId,

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);
   }

src/renderers/EventRenderer.ts

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