Implement serialization/deserialization and persistent storage across services

- Add serialize/deserialize support for core models
- Add file-based load/save functions in management services
- Introduce FileManager, Config, Utility and helper utilities
- Persist observer IDs for notification services
- Resolve object relationships during load (services, bookings, invoices, job cards)
- Add controller-level loadSystemData/saveSystemData
- Load data at app startup and save on shutdown

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/views/UserInterface.cpp

@@ -4,6 +4,7 @@
 
 void UserInterface::run()
 {
+	m_controller.loadSystemData();
 	bool isMenuActive = true;
 	while (isMenuActive)
 	{
@@ -24,6 +25,7 @@ void UserInterface::run()
 			util::pressEnter();
 		}
 	}
+	m_controller.saveSystemData();
 }
 
 bool UserInterface::handleOperation(int choice)