fix(player): size volume slider handle proportionally

lib/widgets/app_slider.dart

@@ -16,6 +16,7 @@ class AppSlider extends StatelessWidget {
     this.label,
     this.semanticFormatterCallback,
     this.trackThickness,
+    this.thumbLength,
   });
 
   final double value;
@@ -31,6 +32,10 @@ class AppSlider extends StatelessWidget {
   /// default extra-small track.
   final double? trackThickness;
 
+  /// Expressive handle height; shorten it together with a thinner track so
+  /// the handle stays in proportion.
+  final double? thumbLength;
+
   @override
   Widget build(BuildContext context) {
     if (!materialExpressiveEnabled(context)) {
@@ -54,6 +59,7 @@ class AppSlider extends StatelessWidget {
         label: label,
         semanticFormatterCallback: semanticFormatterCallback,
         trackThickness: trackThickness,
+        thumbLength: thumbLength,
         enabled: onChanged != null,
         onChanged: onChanged,
         onChangeEnd: onChangeEnd,

lib/widgets/player_volume_control.dart

@@ -206,8 +206,9 @@ class _PlayerVolumeControlState extends ConsumerState<PlayerVolumeControl> {
                 onPointerCancel: (_) => release(),
                 child: AppSlider(
                   // Thinner than the default track: volume stays secondary
-                  // to the seek bar above it.
+                  // to the seek bar above it. The handle shrinks with it.
                   trackThickness: 8,
+                  thumbLength: 24,
                   value: value,
                   onChanged: enabled ? _setVolume : null,
                   onChangeEnd: _setVolume,