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/MixDetail.razor

@@ -41,7 +41,7 @@ else
                         TrackId="@ViewModel.Track?.Id"
                         TrackEntryKey="@ViewModel.Track?.EntryKey" />
 
-    <div class="mix-detail-foreground">
+    <div class="mix-detail-foreground dd-detail-fill">
       <MudContainer MaxWidth="MaxWidth.Large" Class="mix-detail-container">
         @* Mix keeps the scaffold solely for the Phase 10 top row (back link | controls | lava-lamp).
            Title/artist/genre/date/share/play all move into the overlaid hero, so the scaffold's
@@ -60,7 +60,9 @@ else
                    visible in Theater Mode — controls over the experience, not release content (§4/OQ4).
                    Wrapped so they cluster on the right rather than spreading across the SpaceBetween row. *@
                 <div class="dd-detail-top-actions">
-                    <TheaterModeToggle />
+                    @* Theater toggle only appears when this Mix 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, top-right across from the back link (Phase 12
                        §3d-revised). Replaces the former inline TopContent knob-bar: the icon IS the toggle
                        and the popover IS the panel. Mix takes the cleanest anchor case (§8e) — the popover's
@@ -69,10 +71,10 @@ else
                 </div>
             </TopRightAction>
             <Hero>
-                @* Theater Mode (Phase 20 §4): the hero overlay (Session/Mix release content) is removed
-                   from the render so the full-bleed visualizer fills the surface. OFF restores it. *@
-                @if (!VisualizerControlState.TheaterMode)
-                {
+                @* Theater Mode (Phase 20 §4, Wave 2 §2): the hero overlay stays mounted and eases out via
+                   a collapsing wrapper so it does not pop — collapsed to zero height when Theater is on AND
+                   this Mix is the playing release. OFF eases the full-bleed visualizer back behind the hero. *@
+                <div class="dd-theater-collapsible @(IsContentHidden ? "dd-theater-collapsed" : null)">
                 @* Cover-as-background hero with all metadata overlaid, square `mix-hero` sizing. The
                    cover art IS the background, so no separate cover thumbnail (CoverThumbKey defaults
                    to null). Share and play ride in as slots, matching Sessions. *@
@@ -96,14 +98,14 @@ else
                         }
                     </PlayContent>
                 </ReleaseHeroOverlay>
-                }
+                </div>
             </Hero>
             <BodyContent>
-                @if (!VisualizerControlState.TheaterMode)
-                {
+                @* Theater Mode (Wave 2 §2): eased collapse, mirroring the Hero region. *@
+                <div class="dd-theater-collapsible @(IsContentHidden ? "dd-theater-collapsed" : null)">
                     @* Blurb sits below the hero, inside the scaffold's foreground stacking context. *@
                     <ReleaseDescription Description="@release.Description" />
-                }
+                </div>
             </BodyContent>
         </ReleaseDetailScaffold>
       </MudContainer>
@@ -113,7 +115,7 @@ else
 @code {
     protected override string PersistKey => "mix-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; }
 
     // The hero now carries the play affordance (the scaffold's header is suppressed), so the