refactor: share low-risk UI and tracker helpers

lib/utils/scroll_utils.dart

@@ -35,6 +35,21 @@ void scrollToCurrentItem(ScrollController controller, GlobalKey firstItemKey, in
   });
 }
 
+/// Owns the boilerplate for one-time initial scrolling in selectable lists.
+class InitialItemScrollController {
+  final GlobalKey firstItemKey = GlobalKey();
+  final ScrollController controller = ScrollController();
+  bool _didInitialScroll = false;
+
+  void maybeScrollTo(int? selectedIndex) {
+    if (_didInitialScroll || selectedIndex == null || selectedIndex <= 0) return;
+    _didInitialScroll = true;
+    scrollToCurrentItem(controller, firstItemKey, selectedIndex);
+  }
+
+  void dispose() => controller.dispose();
+}
+
 /// Scroll a horizontal list to center the item at the given index.
 ///
 /// Assumes items are laid out with [leadingPadding] before the first item,