Improves drag cancellation behavior

Implements drag cancellation when the mouse leaves the calendar container during a drag operation. This prevents orphaned drag clones and restores the original event's state, enhancing user experience.

All-day events now correctly recalculate their height upon drag cancellation, ensuring accurate rendering after clone removal.

Refactors HeaderManager by removing redundant caching of the calendar header element.

Adds new mock event data for September and October 2025 to expand testing and demonstration scenarios.

src/managers/AllDayManager.ts

@@ -107,6 +107,19 @@ export class AllDayManager {
       console.log('🎯 AllDayManager: Ending drag for all-day event', { eventId });
       this.handleDragEnd(draggedElement, dragClone as HTMLElement, finalPosition.column);
     });
+
+    // Listen for drag cancellation to recalculate height
+    eventBus.on('drag:cancelled', (event) => {
+      const { draggedElement, reason } = (event as CustomEvent).detail;
+      
+      console.log('🚫 AllDayManager: Drag cancelled', {
+        eventId: draggedElement?.dataset?.eventId,
+        reason
+      });
+      
+      // Recalculate all-day height since clones may have been removed
+      this.checkAndAnimateAllDayHeight();
+    });
   }
 
   /**