feat(flutter/library): card + row widgets (artist/album/track)

HorizontalScrollRow takes an optional shared ScrollController so
multi-row sections (Most played: 3 rows) couple their scroll like the
web HorizontalScrollRow.svelte. Cards fall back to the synced
album-fallback.svg when coverUrl is empty.

flutter_client/lib/library/widgets/horizontal_scroll_row.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+
+import '../../theme/theme_extension.dart';
+
+/// Mirrors the web HorizontalScrollRow: a labeled section that scrolls
+/// horizontally. Multi-row sections share one [controller] so they
+/// scroll together.
+class HorizontalScrollRow extends StatelessWidget {
+  const HorizontalScrollRow({
+    required this.title,
+    required this.children,
+    this.height = 200,
+    this.controller,
+    super.key,
+  });
+
+  final String title;
+  final List<Widget> children;
+  final double height;
+  final ScrollController? controller;
+
+  @override
+  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,
+          ),
+        ),
+      ),
+      SizedBox(
+        height: height,
+        child: ListView(
+          controller: controller,
+          scrollDirection: Axis.horizontal,
+          padding: const EdgeInsets.symmetric(horizontal: 16),
+          children: children,
+        ),
+      ),
+    ]);
+  }
+}