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/VolumeZone.razor.css

@@ -0,0 +1,15 @@
+/* Width caps only — layout/colour come from MudStack + theme.
+   The zone stacks the spectrum visualizer above the volume row. Width is sized
+   so 24 spectrum bars (4px min + 2px gap ≈ 140px) render without clipping under
+   the container's overflow:hidden, while staying compact. */
+.volume-zone {
+    width: 150px;
+}
+
+.volume-row {
+    width: 100%;
+}
+
+.volume-slider {
+    flex: 1;
+}