Implement View Invoices for Customer

<UserStory> PAY003: View Invoices </UserStory>

<Changes>
    1. Added CustomerMenu::viewInvoices to fetch invoices via Controller and display them to the customer.
    2. Implemented displayInvoices helper to show invoice details including booking, vehicle, technician, discount, total amount, invoice date, and payment status.
    3. Enhanced invoice display to include a tabular breakdown of inventory items (ItemName, Quantity, Price) used in the service.
    4. Updated ServiceBooking to store assigned technician as a const User* for richer technician details in invoice output.
</Changes>

<Test>

 Acceptance Criteria:
 1. Invoice details should show total cost and discounts.
 2. Invoice details should show list of parts used.

 Precondition:
  1. Customer is logged into the system.
  2. At least one invoice exists for the customer in the datastore.
  3. Inventory items are linked to the invoice.

 Steps:
  1. Navigate to Customer menu and choose "View Invoices".
    - Verify that the system lists invoice details including booking ID, vehicle info, technician, discount, total amount, invoice date, and payment status.
  2. Check the parts list under each invoice.
    - Verify that inventory items are displayed in tabular form with ItemName, Quantity, and Price.
  3. Confirm that discounts and total cost are shown correctly.
</Test>

<Review>
Sreeja Reghukumar, please review
</Review>

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

@@ -1,6 +1,12 @@
+#include <iomanip>
 #include "CustomerMenu.h"
 #include "InputHelper.h"
 #include "OutputHelper.h"
+#include "Invoice.h"
+#include "Service.h"
+#include "ServiceBooking.h"
+#include "User.h"
+#include "InventoryItem.h"
 
 void CustomerMenu::showMenu()
 {
@@ -59,8 +65,62 @@ void CustomerMenu::completePayments()
 {
 }
 
+static void displayInvoices(util::Map<std::string, const Invoice*> currentUserInvoices)
+{
+	if (currentUserInvoices.getSize() == 0)
+	{
+		std::cout << "No invoices found for this account." << std::endl;
+		util::pressEnter();
+		return;
+	}
+	else
+	{
+        for (int index = 0; index < currentUserInvoices.getSize(); index++)
+        {
+            const Invoice* currentInvoice = currentUserInvoices.getValueAt(index);
+            if (currentInvoice)
+            {
+                std::cout << "\nInvoice Details\n";
+                std::cout << "Booking ID: " << currentInvoice->getBookingId() << std::endl;
+                std::cout << "Vehicle Brand: " << currentInvoice->getBooking()->getVehicleBrand() << std::endl;
+                std::cout << "Vehicle Number: " << currentInvoice->getBooking()->getVehicleNumber() << std::endl;
+                std::cout << "Technician ID: " << currentInvoice->getBooking()->getAssignedTechnician()->getId() << std::endl;
+                std::cout << "Technician Name: " << currentInvoice->getBooking()->getAssignedTechnician()->getName() << std::endl;
+                std::cout << "Discount(%): " << currentInvoice->getDiscountPercentage() << std::endl;
+                std::cout << "Total Amount: " << currentInvoice->getTotalAmount() << std::endl;
+                std::cout << "Invoice Date: " << currentInvoice->getInvoiceDate().toString() << std::endl;
+                std::cout << "Payment Status: " << util::getPaymentStatusString(currentInvoice->getStatus()) << std::endl;
+                auto inventoryItemsInInvoice = currentInvoice->getParts();
+                std::cout << "\nItems Used:\n";
+                std::cout << std::left
+                    << std::setw(20) << "ItemName"
+                    << std::setw(10) << "Quantity"
+                    << std::setw(10) << "Price"
+                    << std::endl;
+                std::cout << std::string(40, '-') << std::endl;
+                for (int iterator = 0; iterator < inventoryItemsInInvoice.getSize(); iterator++)
+                {
+                    InventoryItem* currentItem = inventoryItemsInInvoice.getValueAt(iterator);
+                    std::cout << std::left
+                        << std::setw(20) << currentItem->getPartName()
+                        << std::setw(10) << currentItem->getQuantity()
+                        << std::setw(10) << currentItem->getPrice()
+                        << std::endl;
+                }
+            }
+            else
+            {
+                throw std::runtime_error("Null invoice encountered while displaying invoices.");
+            }
+        }
+	}
+}
+
 void CustomerMenu::viewInvoices()
 {
+	util::clear();
+	util::Map<std::string, const Invoice*> currentUserInvoices = m_controller.getInvoicesByUser();
+	displayInvoices(currentUserInvoices);
 }
 
 void CustomerMenu::viewNotifications()