Fixes drag and drop to header issues

Addresses two key issues related to dragging events to the header: the premature removal of the original event element and the creation of duplicate all-day events.

The original event is no longer removed when dragging to the header; it is now only removed upon a successful drop.

Also, it prevents the creation of duplicate all-day events by checking for existing all-day events before creating new ones, using DOM queries to ensure accurate state.

src/managers/ScrollManager.ts

@@ -42,6 +42,16 @@ export class ScrollManager {
       this.updateScrollableHeight();
     });
     
+    // Handle header rebuild - refresh header reference and re-sync
+    eventBus.on('header:rebuilt', () => {
+      this.calendarHeader = document.querySelector('swp-calendar-header');
+      if (this.scrollableContent && this.calendarHeader) {
+        this.setupHorizontalScrollSynchronization();
+        this.syncCalendarHeaderPosition(); // Immediately sync position
+      }
+      this.updateScrollableHeight(); // Update height calculations
+    });
+    
     // Handle window resize
     window.addEventListener('resize', () => {
       this.updateScrollableHeight();