Refactor entity services with hybrid sync pattern

Introduces BaseEntityService and SyncPlugin to eliminate code duplication across entity services

Improves:
- Code reusability through inheritance and composition
- Sync infrastructure for all entity types
- Polymorphic sync status management
- Reduced boilerplate code by ~75%

Supports generic sync for Event, Booking, Customer, and Resource entities

src/types/BookingTypes.ts

@@ -1,3 +1,5 @@
+import { ISync } from './CalendarTypes';
+
 /**
  * Booking entity - represents customer service bookings ONLY
  *
@@ -18,7 +20,7 @@
  *
  * Matches backend Booking table structure
  */
-export interface IBooking {
+export interface IBooking extends ISync {
   id: string;
   customerId: string;        // REQUIRED - booking is always for a customer
   status: BookingStatus;

src/types/CalendarTypes.ts

@@ -9,13 +9,35 @@ export type CalendarView = ViewPeriod;
 
 export type SyncStatus = 'synced' | 'pending' | 'error';
 
+/**
+ * EntityType - Discriminator for all syncable entities
+ */
+export type EntityType = 'Event' | 'Booking' | 'Customer' | 'Resource';
+
+/**
+ * ISync - Interface composition for sync status tracking
+ * All syncable entities should extend this interface
+ */
+export interface ISync {
+  syncStatus: SyncStatus;
+}
+
+/**
+ * IDataEntity - Wrapper for entity data with typename discriminator
+ * Used in queue operations and API calls to preserve type information at runtime
+ */
+export interface IDataEntity {
+  typename: EntityType;
+  data: any;
+}
+
 export interface IRenderContext {
   container: HTMLElement;
   startDate: Date;
   endDate: Date;
 }
 
-export interface ICalendarEvent {
+export interface ICalendarEvent extends ISync {
   id: string;
   title: string;
   description?: string;
@@ -23,7 +45,6 @@ export interface ICalendarEvent {
   end: Date;
   type: CalendarEventType;   // Event type - only 'customer' has associated booking
   allDay: boolean;
-  syncStatus: SyncStatus;
 
   // References (denormalized for IndexedDB performance)
   bookingId?: string;        // Reference to booking (only if type = 'customer')

src/types/CustomerTypes.ts

@@ -1,8 +1,10 @@
+import { ISync } from './CalendarTypes';
+
 /**
  * Customer entity
  * Matches backend Customer table structure
  */
-export interface ICustomer {
+export interface ICustomer extends ISync {
   id: string;
   name: string;
   phone: string;

src/types/ResourceTypes.ts

@@ -1,8 +1,10 @@
+import { ISync } from './CalendarTypes';
+
 /**
  * Resource entity - represents people, rooms, equipment, etc.
  * Matches backend Resource table structure
  */
-export interface IResource {
+export interface IResource extends ISync {
   id: string;              // Primary key (e.g., "EMP001", "ROOM-A")
   name: string;            // Machine name (e.g., "karina.knudsen")
   displayName: string;     // Human-readable name (e.g., "Karina Knudsen")