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/constants/CoreEvents.ts

@@ -19,11 +19,12 @@ export const CoreEvents = {
   PERIOD_INFO_UPDATE: 'nav:period-info-update',
   NAVIGATE_TO_EVENT: 'nav:navigate-to-event',
   
-  // Data events (4)
+  // Data events (5)
   DATA_LOADING: 'data:loading',
   DATA_LOADED: 'data:loaded',
   DATA_ERROR: 'data:error',
   EVENTS_FILTERED: 'data:events-filtered',
+  REMOTE_UPDATE_RECEIVED: 'data:remote-update',
   
   // Grid events (3)
   GRID_RENDERED: 'grid:rendered',
@@ -36,9 +37,16 @@ export const CoreEvents = {
   EVENT_DELETED: 'event:deleted',
   EVENT_SELECTED: 'event:selected',
   
-  // System events (2)
+  // System events (3)
   ERROR: 'system:error',
   REFRESH_REQUESTED: 'system:refresh',
+  OFFLINE_MODE_CHANGED: 'system:offline-mode-changed',
+
+  // Sync events (4)
+  SYNC_STARTED: 'sync:started',
+  SYNC_COMPLETED: 'sync:completed',
+  SYNC_FAILED: 'sync:failed',
+  SYNC_RETRY: 'sync:retry',
   
   // Filter events (1)
   FILTER_CHANGED: 'filter:changed',