refactor: make drag helper start position readonly

Author: mattlewis92

src/modules/common/calendar-drag-helper.provider.ts

@@ -1,7 +1,7 @@
 import { isInside } from './util';
 
 export class CalendarDragHelper {
-  startPosition: ClientRect;
+  private readonly startPosition: ClientRect;
 
   constructor(
     private dragContainerElement: HTMLElement,