feat(library): show a trailing fade on the filter chips row

Extend ScrollEdgeFade with a horizontal axis and wrap the All/Albums/
Singles/Playlists chips so a right-edge fade signals the row scrolls
when chips overflow the screen.

lib/widgets/scroll_edge_fade.dart

@@ -1,12 +1,20 @@
 import 'package:flutter/material.dart';
 
-/// Overlays a bottom gradient on a vertical scrollable while more content
-/// remains below, signalling that the view can scroll.
+/// Overlays a gradient on the trailing edge of a scrollable while more
+/// content remains beyond it, signalling that the view can scroll.
 class ScrollEdgeFade extends StatefulWidget {
   final Widget child;
-  final double height;
+  final Axis axis;
 
-  const ScrollEdgeFade({super.key, required this.child, this.height = 96});
+  /// Thickness of the fade: height for vertical, width for horizontal.
+  final double size;
+
+  const ScrollEdgeFade({
+    super.key,
+    required this.child,
+    this.axis = Axis.vertical,
+    this.size = 96,
+  });
 
   @override
   State<ScrollEdgeFade> createState() => _ScrollEdgeFadeState();
@@ -16,7 +24,7 @@ class _ScrollEdgeFadeState extends State<ScrollEdgeFade> {
   bool _hasMore = false;
 
   bool _onMetrics(ScrollMetrics metrics) {
-    if (metrics.axis == Axis.vertical) {
+    if (metrics.axis == widget.axis) {
       final hasMore = metrics.extentAfter > 8;
       if (hasMore != _hasMore) setState(() => _hasMore = hasMore);
     }
@@ -26,6 +34,7 @@ class _ScrollEdgeFadeState extends State<ScrollEdgeFade> {
   @override
   Widget build(BuildContext context) {
     final background = Theme.of(context).scaffoldBackgroundColor;
+    final horizontal = widget.axis == Axis.horizontal;
     return Stack(
       children: [
         NotificationListener<ScrollMetricsNotification>(
@@ -35,20 +44,26 @@ class _ScrollEdgeFadeState extends State<ScrollEdgeFade> {
             child: widget.child,
           ),
         ),
-        Positioned(
-          left: 0,
-          right: 0,
+        PositionedDirectional(
+          start: horizontal ? null : 0,
+          end: 0,
+          top: horizontal ? 0 : null,
           bottom: 0,
           child: IgnorePointer(
             child: AnimatedOpacity(
               opacity: _hasMore ? 1 : 0,
               duration: const Duration(milliseconds: 200),
               child: Container(
-                height: widget.height,
+                width: horizontal ? widget.size : null,
+                height: horizontal ? null : widget.size,
                 decoration: BoxDecoration(
                   gradient: LinearGradient(
-                    begin: Alignment.topCenter,
-                    end: Alignment.bottomCenter,
+                    begin: horizontal
+                        ? AlignmentDirectional.centerStart
+                        : Alignment.topCenter,
+                    end: horizontal
+                        ? AlignmentDirectional.centerEnd
+                        : Alignment.bottomCenter,
                     colors: [background.withValues(alpha: 0), background],
                   ),
                 ),