fix: Make transactions atomic / AutoLogout

lib/viewmodels/inventory_view_model.dart

@@ -65,4 +65,16 @@ class InventoryViewModel extends ChangeNotifier {
       rethrow;
     }
   }
+
+  void applyStockDelta(String productId, int delta) {
+    if (delta == 0) return;
+
+    final index = _products.indexWhere((product) => product.id == productId);
+    if (index == -1) return;
+
+    _products[index] = _products[index].copyWith(
+      stockQuantity: _products[index].stockQuantity + delta,
+    );
+    notifyListeners();
+  }
 }