Improves event drag and drop highlighting

Enhances the visual feedback during and after event drag and drop operations in the calendar.

- Preserves the full opacity of the dragged event clone during the drag operation for better visibility.
- Applies a highlight class to the event after it's dropped to visually indicate the new location.
- Adds specific styling for the highlighted state based on event type.

src/elements/SwpEventElement.ts

@@ -354,6 +354,9 @@ export class SwpAllDayEventElement extends BaseSwpEventElement {
     // Disable pointer events on clone so it doesn't interfere with hover detection
     clone.style.pointerEvents = 'none';
 
+    // Preserve full opacity during drag
+    clone.style.opacity = '1';
+
     return clone;
   }