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/AudioPlayerBar.razor.css

@@ -74,7 +74,7 @@
 
 /* Unified responsive player layout.
    Wide and narrow shapes are pure CSS — no runtime breakpoint subscription.
-   Children are targeted by their stable classes (transport-zone, volume-controls,
+   Children are targeted by their stable classes (transport-zone, volume-zone,
    seek-zone) rather than positional nth-child, since all three render a .mud-stack
    root and positional selectors would be fragile. */
 .player-layout {
@@ -89,12 +89,12 @@
 @media (min-width: 600px) {
     ::deep .transport-zone { order: 1; }
     ::deep .seek-zone { order: 2; flex-grow: 1; flex-basis: 0; }
-    ::deep .volume-controls { order: 3; }
+    ::deep .volume-zone { order: 3; flex-shrink: 0; }
 }
 
 /* Narrow (< 600px): transport + volume on top row, seek full-width below */
 @media (max-width: 599.98px) {
     ::deep .transport-zone { order: 1; }
-    ::deep .volume-controls { order: 2; }
+    ::deep .volume-zone { order: 2; }
     ::deep .seek-zone { flex-basis: 100%; order: 3; }
 }