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/CalendarManager.ts

@@ -380,6 +380,13 @@ export class CalendarManager {
         
         // Re-render events in the new grid structure
         this.rerenderEvents();
+        
+        // Notify HeaderManager with correct current date after grid rebuild
+        this.eventBus.emit('workweek:header-update', {
+            currentDate: this.currentDate,
+            currentView: this.currentView,
+            workweek: calendarConfig.getCurrentWorkWeek()
+        });
     }
 
     /**