Resolve VSN-1713

Integrates the driver absence management system with the backend API :

Add absence API service layer with CRUD operations Implement document upload for absence justifications Add DatePickerModal component for improved date selection UX Integrate real API endpoints replacing mock data Support multiple absence subtypes (EXTERNAL_ASSIGNMENT, PROFESSIONAL_TRAINING, etc.) Add data mapping between frontend and backend models Implement automatic hours calculation for absences Add comprehensive error handling and fallback mechanisms