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/Controls/AudioPlayerBar/NowShowingPanel.razor

@@ -0,0 +1,46 @@
+@namespace DeepDrftPublic.Client.Controls.AudioPlayerBar
+@using DeepDrftModels.DTOs
+@using DeepDrftPublic.Client.Common
+@using DeepDrftPublic.Client.Controls
+
+@* "Now showing" block surfaced in the player bar when Theater Mode is ON (Phase 20 §5/§7). Theater
+   hides the release page, so the bar carries the release identity the page would have shown: cover art,
+   the release title linked to its detail page, and a release-mode share. Purely presentational — it owns
+   no player logic and no Theater state; AudioPlayerBar mounts it only when state.TheaterMode &&
+   CurrentTrack?.Release is not null, so Release is non-null here.
+
+   Theming is all reuse (§8, zero new CSS): the cover reuses the deepdrft-track-detail-cover-art /
+   -placeholder idiom; the share glyph goes green-accent in both themes via .dd-accent-icon; surface and
+   text come from the bar's own .player-surface and the .now-showing-* classes in the global sheet, which
+   bind the theme-aware --deepdrft-page-* aliases. *@
+
+<div class="now-showing">
+    <div class="now-showing-cover">
+        @if (!string.IsNullOrEmpty(Release.ImagePath))
+        {
+            <div class="deepdrft-track-detail-cover-art now-showing-cover-art"
+                 style="@($"background-image: url('api/image/{Uri.EscapeDataString(Release.ImagePath)}');")"></div>
+        }
+        else
+        {
+            <div class="deepdrft-track-detail-cover-placeholder deepdrft-gradient-soft-secondary now-showing-cover-placeholder">
+                <MudIcon Icon="@Icons.Material.Filled.Album" Color="Color.Primary" />
+            </div>
+        }
+    </div>
+
+    <a href="@ReleaseRoutes.DetailHref(Release)" class="now-showing-title-link">
+        <MudText Typo="Typo.subtitle2" Class="now-showing-title text-truncate">
+            @Release.Title
+        </MudText>
+    </a>
+
+    <div class="dd-accent-icon now-showing-share">
+        <SharePopover ReleaseEntryKey="@Release.EntryKey" ReleaseMedium="@Release.Medium" />
+    </div>
+</div>
+
+@code {
+    /// <summary>The current playing track's release. Non-null by the bar's mount gate.</summary>
+    [Parameter, EditorRequired] public ReleaseDto Release { get; set; } = default!;
+}