feat(frontend) : add notification DTO and service

frontend/services/dto/notification.ts

@@ -0,0 +1,13 @@
+export type NotificationType = 'ticket_created' | 'ticket_status_changed'
+
+export type Notification = {
+    '@id'?: string
+    id: number
+    user: string
+    type: NotificationType
+    title: string
+    message: string
+    relatedTicket: string | null
+    isRead: boolean
+    createdAt: string
+}