Fix GUI tree expansion state

Refactor draw_node signature to draw_node(ctx, &id, node, visible) and add an expanded flag. Only call nk_tree_image_push_ex when visible is true, and only call nk_tree_pop if a tree was actually pushed. Pass the expanded state to child recursive calls so children won't create UI entries when their parent is collapsed. Also renamed local counters (attr_count/child_count) and removed the unused level parameter; updated the top-level caller to pass nk_true.