Implements offline-first calendar sync infrastructure

Adds IndexedDB and operation queue for robust offline synchronization
Introduces SyncManager to handle background data synchronization
Supports local event operations with automatic remote sync queuing

Enhances application reliability and user experience in low/no connectivity scenarios

src/managers/AllDayManager.ts

@@ -127,13 +127,13 @@ export class AllDayManager {
     });
 
     // Listen for header ready - when dates are populated with period data
-    eventBus.on('header:ready', (event: Event) => {
+    eventBus.on('header:ready', async (event: Event) => {
       let headerReadyEventPayload = (event as CustomEvent<IHeaderReadyEventPayload>).detail;
 
       let startDate = new Date(headerReadyEventPayload.headerElements.at(0)!.date);
       let endDate = new Date(headerReadyEventPayload.headerElements.at(-1)!.date);
 
-      let events: ICalendarEvent[] = this.eventManager.getEventsForPeriod(startDate, endDate);
+      let events: ICalendarEvent[] = await this.eventManager.getEventsForPeriod(startDate, endDate);
       // Filter for all-day events
       const allDayEvents = events.filter(event => event.allDay);
 
@@ -380,7 +380,7 @@ export class AllDayManager {
   }
 
 
-  private handleDragEnd(dragEndEvent: IDragEndEventPayload): void {
+  private async handleDragEnd(dragEndEvent: IDragEndEventPayload): Promise<void> {
 
     const getEventDurationDays = (start: string | undefined, end: string | undefined): number => {
 
@@ -496,6 +496,15 @@ export class AllDayManager {
     // 7. Apply highlight class to show the dropped event with highlight color
     dragEndEvent.draggedClone.classList.add('highlight');
 
+    // 8. CRITICAL FIX: Update event in repository to mark as allDay=true
+    // This ensures EventManager's repository has correct state
+    // Without this, the event will reappear in grid on re-render
+    await this.eventManager.updateEvent(eventId, {
+      start: newStartDate,
+      end: newEndDate,
+      allDay: true
+    });
+
     this.fadeOutAndRemove(dragEndEvent.originalElement);
 
     this.checkAndAnimateAllDayHeight();