feat(public): add Theater Mode to release detail pages

Toggle left of the lava popover hides release content so the visualizer fills
the surface; player bar grows to carry the playing release's cover, title, and
share. State on WaveformVisualizerControlState; pages and bar observe it.

DeepDrftPublic.Client/Services/WaveformVisualizerControlState.cs

@@ -97,6 +97,13 @@ public sealed class WaveformVisualizerControlState
     /// </summary>
     public const bool DefaultWaveformEnabled = true;
 
+    /// <summary>
+    /// Default Theater-mode state. <c>false</c> so a fresh page load opens with the full release page,
+    /// not the bare visualizer (Phase 20 §4/OQ5). Has no TS-side anchor: Theater Mode is a page-chrome
+    /// presentation flag, not a visualizer dial — the bridge never reads it.
+    /// </summary>
+    public const bool DefaultTheaterMode = false;
+
     /// <summary>Apparent bottom-to-top scroll rate, normalized [0,1]. Bridge maps it to a visible
     /// time-span via <see cref="WaveformZoomMapping"/>; the standalone resolution/zoom control is gone.</summary>
     public double ScrollSpeed { get; set; } = DefaultScrollSpeed;
@@ -137,9 +144,19 @@ public sealed class WaveformVisualizerControlState
     /// </summary>
     public bool WaveformEnabled { get; set; } = DefaultWaveformEnabled;
 
+    /// <summary>
+    /// Whether Theater Mode is on (Phase 20). When <c>true</c> the three release-detail pages remove
+    /// their release content via <c>@if</c> so the visualizer fills the surface, and the player bar
+    /// grows to carry the playing release's identity. Distinct from the visualizer dials: the bridge
+    /// ignores it — the pages and the player bar observe it through the same <see cref="Changed"/> seam.
+    /// Gated for visibility on <see cref="LavaEnabled"/> || <see cref="WaveformEnabled"/> at the toggle.
+    /// </summary>
+    public bool TheaterMode { get; set; } = DefaultTheaterMode;
+
     /// <summary>
     /// Raised whenever any control value changes. The visualizer bridge subscribes to push the
-    /// affected dial(s). Mutators set the property then raise this; subscribers re-read the values.
+    /// affected dial(s); the Theater-Mode observers (detail pages, player bar) subscribe to react to
+    /// <see cref="TheaterMode"/>. Mutators set the property then raise this; subscribers re-read the values.
     /// </summary>
     public event Action? Changed;