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/Pages/SessionDetail.razor

@@ -49,7 +49,7 @@ else
                         TrackId="@ViewModel.Track?.Id"
                         TrackEntryKey="@ViewModel.Track?.EntryKey" />
 
-    <MudContainer MaxWidth="MaxWidth.Large" Class="session-detail-page session-detail-foreground">
+    <MudContainer MaxWidth="MaxWidth.Large" Class="session-detail-page session-detail-foreground dd-detail-fill">
 
         <div class="session-detail-top-row">
             <MudLink Href="/sessions" Typo="Typo.body2" Class="deepdrft-track-detail-back">
@@ -59,18 +59,19 @@ else
             @* Theater toggle sits immediately LEFT of the lava-lamp popover (Phase 20 §3). The whole top
                row (back + theater + lava) stays in Theater Mode — controls, not release content (§4/OQ4). *@
             <div class="dd-detail-top-actions">
-                <TheaterModeToggle />
+                @* Theater toggle only appears when this Session is the currently-playing release
+                   (Phase 20 Wave 2 §3). ShowTheaterToggle folds in the subsystem + release-playing gate. *@
+                <TheaterModeToggle Available="ShowTheaterToggle" />
                 @* Lava-lamp icon → popover panel (full parity, §3e/§3d-revised). Anchored top-right, clear of
                    the hero overlay and the share/play affordances overlaid on the hero below. *@
                 <WaveformVisualizerControlPopover />
             </div>
         </div>
 
-        @* Theater Mode (Phase 20 §4): the hero overlay + blurb (the session's release content) are removed
-           from the render so the ambient visualizer fills the surface. The top row above stays. OFF
-           restores this region byte-for-byte. *@
-        @if (!VisualizerControlState.TheaterMode)
-        {
+        @* Theater Mode (Phase 20 §4, Wave 2 §2): the hero overlay + blurb stay mounted and ease out via a
+           collapsing wrapper so they do not pop — collapsed to zero height when Theater is on AND this
+           Session is the playing release. The top row above stays. OFF eases this region back in. *@
+        <div class="dd-theater-collapsible @(IsContentHidden ? "dd-theater-collapsed" : null)">
         @* The overlay shows the cover thumbnail only when it differs from the resolved hero image —
            when there is no dedicated hero, heroImage already falls back to release.ImagePath, so the
            thumb would duplicate the background. That logic lives in ReleaseHeroOverlay. *@
@@ -96,7 +97,7 @@ else
         </ReleaseHeroOverlay>
 
         <ReleaseDescription Description="@release.Description" />
-        }
+        </div>
 
     </MudContainer>
 }
@@ -104,7 +105,7 @@ else
 @code {
     protected override string PersistKey => "session-detail";
 
-    [CascadingParameter] public IStreamingPlayerService? PlayerService { get; set; }
+    // PlayerService is cascaded by ReleaseDetailBase (used there for the Theater release-playing predicate).
     [CascadingParameter] public IQueueService? Queue { get; set; }
 
     // Mirrors the play-toggle wiring the shared scaffold owns. Session detail composes the player