feat(flutter/widgets): HorizontalScrollRow skips header when title empty

Lets multiple stacked HorizontalScrollRow instances render under a
single visible heading by passing title: "" to the second-row-onward
instances. Combined with the existing shared-controller support, this
gives us multi-row scrollers without a new widget.

Used in the next commit's home rewrite for Recently added (2 rows)
and Most played (3 rows).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/library/widgets/horizontal_scroll_row.dart

@@ -23,6 +23,7 @@ class HorizontalScrollRow extends StatelessWidget {
   Widget build(BuildContext context) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
+      if (title.isNotEmpty)
         Padding(
           padding: const EdgeInsets.fromLTRB(16, 8, 16, 8),
           child: Text(