reorderPage method
Moves page from oldIndex to newIndex for page reordering.
Implementation
void reorderPage(int oldIndex, int newIndex) {
if (oldIndex < 0 || oldIndex >= _pages.length) return;
if (newIndex < 0 || newIndex >= _pages.length) return;
final page = _pages.removeAt(oldIndex);
_pages.insert(newIndex, page);
}