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/controllers/Controller.h

@@ -2,6 +2,10 @@
 #include "Map.h"
 #include <string>
 #include "Enums.h"
+#include "InventoryManagementService.h"
+#include "UserManagementService.h"
+#include "ServiceManagementService.h"
+#include "PaymentManagementService.h"
 
 class Service;
 class ComboPackage;
@@ -14,6 +18,11 @@ class Notification;
 
 class Controller
 {
+private:
+    UserManagementService m_userManagementService;
+    InventoryManagementService m_inventoryManagementService;
+    ServiceManagementService m_serviceManagementService;
+    PaymentManagementService m_paymentManagementService;
 public:
     bool login(const std::string& username, const std::string& password);
     void logout();
@@ -47,5 +56,7 @@ public:
     util::Vector<const Notification*> getNotifications();
     void deleteNotification(const std::string& notificationID);
     void configureNotifications(const std::string& userID, bool paymentNotifications, bool serviceNotifications);
+    void loadSystemData();
+    void saveSystemData();
     void runSystemChecks();
 };