refactor(player): move SpectrumVisualizer into VolumeZone above volume slider

Rename VolumeControls to VolumeZone; stack 24-bucket SpectrumVisualizer above volume
slider; remove it from PlayerSeekZone. MudSlider stays as seek placeholder. Pin
flex-shrink:0 on volume-zone; add Class param to VolumeZone for layout flexibility.

DeepDrftPublic.Client/Controls/AudioPlayerBar/SpectrumVisualizer.razor.cs

@@ -9,7 +9,7 @@ public partial class SpectrumVisualizer : ComponentBase, IAsyncDisposable
 
     [CascadingParameter] public IStreamingPlayerService? PlayerService { get; set; }
 
-    [Parameter] public int BucketCount { get; set; } = 32;
+    [Parameter] public int BucketCount { get; set; } = 24;
 
     private readonly string _instanceId = Guid.NewGuid().ToString();
     private double[] _spectrumData = Array.Empty<double>();