Refactor calendar datasource architecture

Centralizes date calculation logic into DateColumnDataSource
Improves separation of concerns across rendering components

Key changes:
- Introduces IColumnInfo interface for flexible column data
- Moves date calculation from multiple managers to dedicated datasource
- Removes duplicate date rendering logic
- Prepares architecture for future resource-based views

src/managers/DragDropManager.ts

@@ -738,7 +738,7 @@ export class DragDropManager {
     }
 
     const dragMouseLeavePayload: IDragMouseLeaveHeaderEventPayload = {
-      targetDate: targetColumn.date,
+      targetColumn: targetColumn,
       mousePosition: position,
       originalElement: this.originalElement,
       draggedClone: this.draggedClone