fix: cleanup and small improvements

- added empty check in inventory alerts
- corrected parts cost calculation
- removed unused getUser method
- updated default admin password
- fixed missing includes in menus
- cleaned up MenuHelper comments

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/views/CustomerMenu.cpp

@@ -10,8 +10,10 @@ Date:19-May-2026
 #include <iomanip>
 #include "ComboPackage.h"
 #include "CustomerMenu.h"
+#include "MenuHelper.h"
 #include "Enums.h"
 #include "InputHelper.h"
+#include "OutputHelper.h"
 #include "InventoryItem.h"
 #include "Invoice.h"
 #include "Map.h"