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

@@ -23,17 +23,18 @@ class HorizontalScrollRow extends StatelessWidget {
   Widget build(BuildContext context) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
-      Padding(
-        padding: const EdgeInsets.fromLTRB(16, 8, 16, 8),
-        child: Text(
-          title,
-          style: TextStyle(
-            fontFamily: fs.display.fontFamily,
-            fontSize: 18,
-            color: fs.parchment,
+      if (title.isNotEmpty)
+        Padding(
+          padding: const EdgeInsets.fromLTRB(16, 8, 16, 8),
+          child: Text(
+            title,
+            style: TextStyle(
+              fontFamily: fs.display.fontFamily,
+              fontSize: 18,
+              color: fs.parchment,
+            ),
           ),
         ),
-      ),
       SizedBox(
         height: height,
         child: ListView(