Refactor calendar datasource and event management

Enhances calendar flexibility by introducing group-based column spanning and improving cross-mode event handling

Adds support for:
- Dynamic column grouping in date and resource modes
- Consistent event drag-and-drop across different calendar views
- More robust all-day event layout calculations

Improves event management logic to handle resource and date mode transitions more elegantly

src/types/EventTypes.ts

@@ -43,6 +43,8 @@ export interface IDragEndEventPayload {
   originalSourceColumn: IColumnBounds;  // Original column where drag started
   finalPosition: {
     column: IColumnBounds | null;  // Where drag ended
+    date: Date;                     // Always present - the date for this position
+    resourceId?: string;            // Only in resource mode
     snappedY: number;
   };
   target: 'swp-day-column' | 'swp-day-header' | null;