added product import and billing integration

lib/presentation/receipt/screens/receipt_screen.dart

@@ -15,13 +15,22 @@ import '../../../core/utils/extensions.dart';
 import '../../../core/utils/formatters.dart';
 import '../../../core/widgets/glass_card.dart';
 import '../../../core/widgets/primary_button.dart';
+import '../../../data/sync/sync_engine.dart';
 import '../../../domain/entities/transaction.dart';
 import '../../modules/providers/printer_settings.dart';
 import '../../pos/providers/cart_controller.dart';
+import '../../pos/providers/catalog_providers.dart';
+import '../../sync/providers/sync_controller.dart';
 import '../widgets/receipt_preview.dart';
 
-/// Confirmation screen. Counts down and starts the next sale on its own so an
-/// unattended terminal never sits on a finished bill.
+/// Confirmation screen.
+///
+/// The bill is written to SQLite the instant checkout completes, but is
+/// deliberately held back from the server for [AppConstants.postSaleResetDelay]
+/// — long enough to catch a mistake. Counts down and starts the next sale (and
+/// releases the bill to the sync engine) on its own so an unattended terminal
+/// never sits on a finished bill forever; "Cancel sale" inside the window
+/// voids it instead and hands the cart straight back.
 class ReceiptScreen extends ConsumerStatefulWidget {
   const ReceiptScreen({super.key, required this.transaction});
 
@@ -32,9 +41,13 @@ class ReceiptScreen extends ConsumerStatefulWidget {
 }
 
 class _ReceiptScreenState extends ConsumerState<ReceiptScreen> {
-  late int _seconds = AppConstants.postSaleResetDelay.inSeconds + 5;
+  late int _seconds = AppConstants.postSaleResetDelay.inSeconds;
   Timer? _timer;
 
+  /// True while a cancellation is being written to disk — guards against a
+  /// second tap voiding a sale that is already half-reversed.
+  bool _voiding = false;
+
   @override
   void initState() {
     super.initState();
@@ -82,13 +95,36 @@ class _ReceiptScreenState extends ConsumerState<ReceiptScreen> {
 
   void _newSale() {
     _timer?.cancel();
+    // This is the one moment the bill is actually released to the server —
+    // whether the window ran out on its own or New Sale was pressed early,
+    // the cashier has let this bill stand.
+    ref.read(syncEngineProvider).nudge(SyncTrigger.saleCommitted);
     ref.read(cartControllerProvider.notifier).reset();
     if (mounted) context.go(AppRoutes.pos);
   }
 
-  void _continueBilling() {
+  /// Undoes the sale: deletes the order, puts the stock back, restores an
+  /// attached shopper's loyalty balance, and hands the exact same cart back
+  /// to the billing screen. Never touches the sync engine — this bill must
+  /// never reach the server.
+  Future<void> _cancelSale() async {
+    if (_voiding) return;
+    setState(() => _voiding = true);
     _timer?.cancel();
-    ref.read(cartControllerProvider.notifier).reset();
+
+    final txn = widget.transaction;
+    await ref.read(transactionRepositoryProvider).voidSale(
+          transaction: txn,
+          stockMovements: {
+            for (final line in txn.cart.lines) line.product.id: line.quantity,
+          },
+        );
+
+    ref.invalidate(allProductsProvider);
+    ref.invalidate(visibleProductsProvider);
+    ref.read(orderVersionProvider.notifier).state++;
+    ref.read(cartControllerProvider.notifier).restore(txn.cart);
+
     if (mounted) context.go(AppRoutes.pos);
   }
 
@@ -293,12 +329,23 @@ class _ReceiptScreenState extends ConsumerState<ReceiptScreen> {
                 : 'New Sale',
             icon: Icons.add_shopping_cart_rounded,
             large: true,
-            onPressed: _newSale,
+            onPressed: _voiding ? null : _newSale,
           ),
           const SizedBox(height: AppSpacing.sm),
-          TextButton(
-            onPressed: _continueBilling,
-            child: const Text('Back to billing screen'),
+          TextButton.icon(
+            onPressed: _voiding ? null : _cancelSale,
+            icon: _voiding
+                ? const SizedBox(
+                    width: 14,
+                    height: 14,
+                    child: CircularProgressIndicator(
+                      strokeWidth: 2,
+                      color: AppColors.danger,
+                    ),
+                  )
+                : const Icon(Icons.undo_rounded, size: 16),
+            label: Text(_voiding ? 'Cancelling…' : 'Cancel sale'),
+            style: TextButton.styleFrom(foregroundColor: AppColors.danger),
           ),
         ],
       ),