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/Pages/CutDetail.razor

@@ -54,11 +54,21 @@ else
                                 TrackEntryKey="@firstTrack?.EntryKey" />
         </Ambient>
         <TopRightAction>
-            @* Lava-lamp icon → popover panel (full parity, §3d-revised). Sits top-right across from the
-               back link, clear of the header's own Play/Share affordances below. *@
-            <WaveformVisualizerControlPopover />
+            @* Theater toggle sits immediately LEFT of the lava-lamp popover (Phase 20 §3). Both are
+               controls over the experience, not release content, so both stay in Theater Mode (§4/OQ4).
+               Wrapped so they cluster on the right rather than spreading across the SpaceBetween row. *@
+            <div class="dd-detail-top-actions">
+                <TheaterModeToggle />
+                @* Lava-lamp icon → popover panel (full parity, §3d-revised). Sits top-right across from the
+                   back link, clear of the header's own Play/Share affordances below. *@
+                <WaveformVisualizerControlPopover />
+            </div>
         </TopRightAction>
         <Header>
+            @* Theater Mode (Phase 20 §4): the release content is removed from the render — not
+               CSS-hidden — so the visualizer fills the surface. OFF restores it byte-for-byte. *@
+            @if (!VisualizerControlState.TheaterMode)
+            {
             @* Header split: meta + Play/Share on the LEFT, bordered cover on the RIGHT (spec §3.1). *@
             <div class="cut-detail-header">
                 <div class="cut-detail-meta">
@@ -117,8 +127,11 @@ else
                     }
                 </div>
             </div>
+            }
         </Header>
         <BodyContent>
+            @if (!VisualizerControlState.TheaterMode)
+            {
             @* Blurb sits between the header and the track-list divider. *@
             <ReleaseDescription Description="@release.Description" />
             <MudDivider Class="cut-detail-divider" />
@@ -149,6 +162,7 @@ else
                     }
                 </div>
             }
+            }
         </BodyContent>
     </ReleaseDetailScaffold>
 }