Adds drag-drop support for calendar events

Introduces comprehensive drag-drop functionality for calendar events
- Implements DragDropManager to handle event dragging
- Adds new CoreEvents for drag-drop interactions
- Supports smooth interpolation and grid snapping
- Provides flexible event handling with ghost element strategy

wwwroot/css/v2/calendar-v2-events.css

@@ -35,10 +35,17 @@ swp-day-columns swp-event {
   &.dragging {
     position: absolute;
     z-index: 999999;
-    opacity: 0.8;
     left: 2px;
     right: 2px;
     width: auto;
+    cursor: grabbing;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+  }
+
+  /* Ghost clone (stays in original position during drag) */
+  &.drag-ghost {
+    opacity: 0.3;
+    pointer-events: none;
   }
 
   /* Hover state */