feat(detail): medium release cover thumbnails on mix and session detail pages

DeepDrftPublic.Client/Pages/SessionDetail.razor

@@ -37,6 +37,10 @@ else
     var heroKey = release.SessionMetadata?.HeroImageEntryKey;
     // Hero image precedence: the session's dedicated hero, then the release cover, then a placeholder.
     var heroImage = !string.IsNullOrEmpty(heroKey) ? heroKey : release.ImagePath;
+    // Show the cover thumbnail only when it differs from what the hero displays. When there is no
+    // dedicated hero, the hero already falls back to release.ImagePath — rendering the cover too
+    // would duplicate the same image.
+    var showCover = !string.IsNullOrEmpty(release.ImagePath) && release.ImagePath != heroImage;
     var hasGenre = release.Genre is not null;
     var hasDate = release.ReleaseDate is not null;
 
@@ -60,6 +64,13 @@ else
                     </MudPaper>
                 }
             </div>
+            @if (showCover)
+            {
+                <div class="session-detail-cover">
+                    <MudPaper Elevation="2" Class="deepdrft-track-detail-cover-art"
+                              Style="@($"background-image: url('api/image/{Uri.EscapeDataString(release.ImagePath!)}');")" />
+                </div>
+            }
         </Hero>
         <MetaContent>
             @if (hasGenre)