fix(music): replace the Instant Mix faders with a wand icon

The three vertical faders read as an equalizer in a music context and
are the vertical twin of the video player's settings icon. Use
wand_stars, which names what the action produces and collides with no
neighbouring affordance in the action bar or the music context menu.

close #1629

lib/widgets/media_context_menu.dart

@@ -395,7 +395,7 @@ class MediaContextMenuState extends State<MediaContextMenu> {
         // reachable (capabilities stay truthy for offline servers).
         if (itemServerOnline && (mediaClient?.capabilities.instantMix ?? false)) {
           menuActions.add(
-            _MenuAction(value: 'music_instant_mix', icon: Symbols.instant_mix_rounded, label: t.music.instantMix),
+            _MenuAction(value: 'music_instant_mix', icon: Symbols.wand_stars_rounded, label: t.music.instantMix),
           );
         }
 

lib/widgets/music/music_actions.dart

@@ -33,7 +33,7 @@ List<FocusableAction> buildMusicActions({
     if (onInstantMix != null)
       FocusableAction(
         debugLabel: 'music_instant_mix',
-        icon: Symbols.instant_mix_rounded,
+        icon: Symbols.wand_stars_rounded,
         tooltip: t.music.instantMix,
         onPressed: onInstantMix,
       ),