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

wwwroot/data/mock-events.json

@@ -4140,6 +4140,7 @@
   {
     "id": "RES-NOV25-001",
     "title": "Balayage kort hår",
+    "description": "Daily team sync - status updates",
     "start": "2025-11-25T09:00:00Z",
     "end": "2025-11-25T10:30:00Z",
     "type": "customer",