pos changes

lib/presentation/pos/providers/cart_controller.dart

@@ -208,10 +208,19 @@ class CartController extends StateNotifier<Cart> {
     setQuantity(productId, line.quantity + by);
   }
 
+  /// Steps a line down, never off the bill.
+  ///
+  /// Floors at one deliberately. Taking the last unit away is a removal, and
+  /// removals go through the PIN gate on the close button — a stepper that
+  /// quietly reached zero was a way around it.
   void decrement(String productId, {double by = 1}) {
     final line = state.lineFor(productId);
     if (line == null) return;
-    setQuantity(productId, line.quantity - by);
+
+    final next = line.quantity - by;
+    if (next < 1) return;
+
+    setQuantity(productId, next);
   }
 
   void removeLine(String productId) {