feat(theater): full-screen detail body, eased content collapse, playing-release scoping

Detail bodies fill 100vh below the nav so the visualizer reads full-screen; Theater toggle eases page content and the player-bar now-showing panel in/out instead of popping (reduced-motion honored); Theater only applies to the currently-playing release.

DeepDrftPublic.Client/Controls/AudioPlayerBar/AudioPlayerBar.razor

@@ -10,13 +10,18 @@ else
         <MudContainer MaxWidth="MaxWidth.Large" Class="player-inner-container">
             <MudPaper Elevation="8" Class="player-surface pa-3">
 
-                @* Theater Mode "now showing" band (Phase 20 §5/§7). Shown only when Theater is ON and a
-                   release is playing — keyed off the playing track's Release, not off any detail page
-                   (the bar reaches into no page; §6). The release page is hidden in Theater Mode, so the
-                   bar carries its identity: cover, linked title, release share. *@
-                @if (VisualizerControlState.TheaterMode && CurrentTrack?.Release is not null)
+                @* Theater Mode "now showing" band (Phase 20 §5/§7, Wave 2 §2). Keyed off the playing
+                   track's Release, not off any detail page (the bar reaches into no page; §6). The release
+                   page is hidden in Theater Mode, so the bar carries its identity: cover, linked title,
+                   release share. The band stays mounted whenever a release is playing and eases in/out via
+                   the shared .dd-theater-collapsible wrapper — collapsed (zero height, faded) unless
+                   Theater is ON — so the bar grows/shrinks smoothly instead of popping. *@
+                @if (CurrentTrack?.Release is not null)
                 {
-                    <NowShowingPanel Release="CurrentTrack.Release" />
+                    var nowShowing = VisualizerControlState.TheaterMode;
+                    <div class="dd-theater-collapsible @(nowShowing ? null : "dd-theater-collapsed")">
+                        <NowShowingPanel Release="CurrentTrack.Release" />
+                    </div>
                 }
 
                 <div class="player-layout">

DeepDrftPublic.Client/Controls/TheaterModeToggle.razor

@@ -10,11 +10,13 @@
    reaches into no page and no bar — single source, multiple observers (§6).
 
    Visible only when the lava OR waveform subsystem is on — there is nothing to go to theater FOR if both
-   are off (§3.2). Disabled until interactive (§3.4), the same prerender guard the lava/Play buttons use.
-   Active visual state when Theater is ON. .dd-accent-icon gives the green-accent glyph in both themes
-   with zero new CSS (§8) — same treatment as the lava-lamp trigger it sits beside. *@
+   are off (§3.2) — AND when <see cref="Available"/> is true. The page supplies Available so the toggle
+   only appears when this page's release is the one playing (Phase 20 Wave 2 §3): the toggle owns the
+   subsystem gate; the page owns the release-playing predicate. Disabled until interactive (§3.4), the
+   same prerender guard the lava/Play buttons use. Active visual state when Theater is ON. .dd-accent-icon
+   gives the green-accent glyph in both themes with zero new CSS (§8) — same as the lava-lamp trigger. *@
 
-@if (State.LavaEnabled || State.WaveformEnabled)
+@if (Available && (State.LavaEnabled || State.WaveformEnabled))
 {
     <div class="dd-accent-icon">
         <MudTooltip Text="@(State.TheaterMode ? "Exit theater mode" : "Theater mode")">
@@ -33,6 +35,14 @@
     /// <summary>Trigger-icon size. Defaults Large to match the lava-lamp popover trigger it sits beside.</summary>
     [Parameter] public Size IconSize { get; set; } = Size.Large;
 
+    /// <summary>
+    /// Whether the toggle is available on this surface (Phase 20 Wave 2 §3). The page passes the
+    /// "this release is the one playing" predicate here; Theater Mode only applies to the playing
+    /// release, so a detail page whose release is not playing passes <c>false</c> and shows no toggle.
+    /// Defaults <c>true</c> so surfaces with no release-scoping (none today) keep the subsystem-only gate.
+    /// </summary>
+    [Parameter] public bool Available { get; set; } = true;
+
     protected override void OnInitialized() => State.Changed += OnStateChanged;
 
     // The toggle's own visibility and active state both key off State, which another observer (or this