Enables all-day event drag and drop

Implements comprehensive drag and drop for all-day events, allowing movement within the header and conversion to timed events when dragged into the calendar grid.

Optimizes column detection with a cached bounding box strategy, improving performance and accuracy. Refactors event conversion logic and renames related event bus events for clarity.

refactored-header-manager.md

@@ -133,7 +133,7 @@ eventBus.on('header:mouseover', (event) => {
     if (draggedElement) {
       console.log('🎯 Converting to all-day for date:', targetDate);
       
-      this.eventBus.emit('drag:convert-to-allday', {
+      this.eventBus.emit('drag:convert-to-allday_event', {
         targetDate,
         originalElement: draggedElement,
         headerRenderer: (event as CustomEvent).detail.headerRenderer