added product import and billing integration

lib/presentation/pos/providers/cart_controller.dart

@@ -302,6 +302,15 @@ class CartController extends StateNotifier<Cart> {
   }
 
   Future<void> resume(ParkedBill bill) async {
+    // Resuming a bill on top of an already-active cart would otherwise
+    // silently overwrite whatever was already there — picking a second
+    // parked bill while the first one's items are still sitting in the
+    // cart, unsaved. Same hole startNewSale closes for the header button;
+    // this closes it here by parking what's active first.
+    if (state.isNotEmpty) {
+      await park(label: 'Auto-parked — replaced by resuming another bill');
+    }
+
     await _transactions.removeParked(bill.id);
     _undoStack.clear();
     // Re-evaluated rather than restored: a campaign that has since ended must
@@ -309,6 +318,32 @@ class CartController extends StateNotifier<Cart> {
     _commit(bill.cart);
   }
 
+  /// What the header's "New Sale" button actually calls.
+  ///
+  /// A cashier can always start over — that's the whole point of the escape
+  /// hatch — but silently wiping a non-empty cart here would be the exact
+  /// same hole the removal PIN closes: scan an item, then abandon the cart
+  /// instead of removing that one line, and it disappears just the same,
+  /// with nobody having approved anything. So a non-empty cart is parked,
+  /// not discarded — visible and resumable from Parked bills — and only an
+  /// already-empty cart takes the plain reset path.
+  Future<void> startNewSale() async {
+    if (state.isNotEmpty) {
+      await park(label: 'Auto-parked — new sale started with items still in cart');
+      return;
+    }
+    reset();
+  }
+
+  /// Puts a cart back exactly as it was just before a sale that has since
+  /// been voided, so cancelling a completed bill doesn't make the cashier
+  /// re-scan everything. Promos are re-evaluated for the same reason
+  /// [resume] re-evaluates them, not restored verbatim.
+  void restore(Cart cart) {
+    _undoStack.clear();
+    _commit(cart);
+  }
+
   List<CartLine> _replace(CartLine updated) => [
         for (final l in state.lines)
           if (l.product.id == updated.product.id) updated else l,