fix(theater): replace max-height collapse with grid-rows + visibility; fix keyboard-focus leak when collapsed

DeepDrftPublic.Client/Pages/MixDetail.razor

@@ -75,6 +75,7 @@ else
                    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)">
+                <div class="dd-theater-collapsible-inner">
                 @* 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. *@
@@ -99,12 +100,15 @@ else
                     </PlayContent>
                 </ReleaseHeroOverlay>
                 </div>
+                </div>
             </Hero>
             <BodyContent>
                 @* Theater Mode (Wave 2 §2): eased collapse, mirroring the Hero region. *@
                 <div class="dd-theater-collapsible @(IsContentHidden ? "dd-theater-collapsed" : null)">
+                    <div class="dd-theater-collapsible-inner">
                     @* Blurb sits below the hero, inside the scaffold's foreground stacking context. *@
                     <ReleaseDescription Description="@release.Description" />
+                    </div>
                 </div>
             </BodyContent>
         </ReleaseDetailScaffold>