Fix modifier nesting - don't add intermediate groups

When user customizes an item, add selections directly under the root
item instead of creating an intermediate group node. This matches how
attachDefaultChildren works on the server and prevents duplicate
modifiers showing in KDS.

Before: Double Double → Customize Lettuce → Light, Extra
After:  Double Double → Light, Extra

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/screens/menu_browse_screen.dart

@@ -1165,6 +1165,7 @@ class _MenuBrowseScreenState extends State<MenuBrowseScreen> {
       final realChildItemId = _decodeVirtualId(child.itemId);
 
       if (isSelected) {
+        // This item is directly selected - add it
         final cart = await Api.setLineItem(
           orderId: orderId,
           parentOrderLineItemId: parentOrderLineItemId,
@@ -1186,21 +1187,13 @@ class _MenuBrowseScreenState extends State<MenuBrowseScreen> {
           );
         }
       } else if (hasSelectedDescendants) {
-        final cart = await Api.setLineItem(
+        // This item is NOT selected, but has selected descendants
-          orderId: orderId,
+        // DON'T add this intermediate group - just recurse to add the actual selections
-          parentOrderLineItemId: parentOrderLineItemId,
+        // The selections will be added directly under the current parent
-          itemId: child.itemId,
+        // This matches how attachDefaultChildren works on the server
-          isSelected: true,
-        );
-
-        final childLineItem = cart.lineItems.lastWhere(
-          (li) => li.itemId == realChildItemId && li.parentOrderLineItemId == parentOrderLineItemId && !li.isDeleted,
-          orElse: () => throw StateError('Failed to add item'),
-        );
-
         await _addModifiersRecursively(
           orderId,
-          childLineItem.orderLineItemId,
+          parentOrderLineItemId,  // Keep the same parent - don't create intermediate node
           child.itemId,
           selectedItemIds,
         );