Migrates date handling from date-fns to day.js

Replaces date-fns library with day.js to reduce bundle size and improve tree-shaking

- Centralizes all date logic in DateService
- Reduces library footprint from 576 KB to 29 KB
- Maintains 99.4% test coverage during migration
- Adds timezone and formatting plugins for day.js

Improves overall library performance and reduces dependency complexity

src/managers/AllDayManager.ts

@@ -19,7 +19,6 @@ import {
 import { IDragOffset, IMousePosition } from '../types/DragDropTypes';
 import { CoreEvents } from '../constants/CoreEvents';
 import { EventManager } from './EventManager';
-import { differenceInCalendarDays } from 'date-fns';
 import { DateService } from '../utils/DateService';
 
 /**
@@ -540,7 +539,7 @@ export class AllDayManager {
     const targetDate = dragEndEvent.finalPosition.column.date;
     
     // Calculate duration in days
-    const durationDays = differenceInCalendarDays(clone.end, clone.start);
+    const durationDays = this.dateService.differenceInCalendarDays(clone.end, clone.start);
     
     // Create new dates preserving time
     const newStart = new Date(targetDate);