Resolve VSN-1943 "Mes demandes"
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