feat(player): add append-only "Add to Queue" buttons beside detail-page play affordances

Cut header (release → EnqueueRange), Cut track rows + Session/Mix hero (track → Enqueue). Reuses existing engine path; add is not play.

DeepDrftPublic.Client/Controls/AddToQueueButton.razor

@@ -0,0 +1,46 @@
+@namespace DeepDrftPublic.Client.Controls
+@using DeepDrftModels.DTOs
+@using DeepDrftPublic.Client.Services
+
+@* Append-only "Add to Queue" affordance placed beside a play control. Add is NOT play: it calls the
+   cascaded IQueueService's Enqueue/EnqueueRange (which append without disturbing current playback and
+   leave a coherent CurrentIndex on a first add into a dormant queue) — never PlayRelease/Start/Select.
+   Track mode (Track set) appends a single track; release mode (ReleaseTracks set) appends the whole
+   ordered list. Reads queue state from the layout-level cascade (C1); owns no data fetch. *@
+
+<MudTooltip Text="@Tooltip">
+    <MudIconButton Icon="@Icons.Material.Filled.PlaylistAdd"
+                   Color="@Color"
+                   Size="@Size"
+                   Disabled="@(Queue is null || !RendererInfo.IsInteractive)"
+                   OnClick="@AddToQueue" />
+</MudTooltip>
+
+@code {
+    [CascadingParameter] public IQueueService? Queue { get; set; }
+
+    /// <summary>Single track to append (track mode). Mutually exclusive with <see cref="ReleaseTracks"/>.</summary>
+    [Parameter] public TrackDto? Track { get; set; }
+
+    /// <summary>Ordered release tracks to append (release mode). Mutually exclusive with <see cref="Track"/>.</summary>
+    [Parameter] public IReadOnlyList<TrackDto>? ReleaseTracks { get; set; }
+
+    [Parameter] public Size Size { get; set; } = Size.Medium;
+    [Parameter] public Color Color { get; set; } = Color.Secondary;
+
+    private string Tooltip => ReleaseTracks is not null ? "Add release to queue" : "Add to queue";
+
+    private void AddToQueue()
+    {
+        if (Queue is null) return;
+
+        if (ReleaseTracks is not null)
+        {
+            Queue.EnqueueRange(ReleaseTracks);
+        }
+        else if (Track is not null)
+        {
+            Queue.Enqueue(Track);
+        }
+    }
+}

DeepDrftPublic.Client/Pages/CutDetail.razor

@@ -94,6 +94,10 @@ else
                             Play
                         </MudButton>
 
+                        @* Append the whole album (TrackNumber order) to the queue — same ordered list
+                           header Play uses. Append-only: does not start playback (AC7/AC8). *@
+                        <AddToQueueButton ReleaseTracks="@ViewModel.Tracks" />
+
                         @* Release-mode share: copies the canonical /cuts/{entryKey} URL, not a single track (§3b). *@
                         <SharePopover ReleaseEntryKey="@release.EntryKey" ReleaseMedium="@release.Medium" />
                     </div>
@@ -138,6 +142,8 @@ else
                                                OnToggle="@(() => PlayTrack(track, index))" />
                             </div>
                             <span class="cut-detail-track-name text-truncate">@track.TrackName</span>
+                            @* Append this single track to the queue (append-only, does not play). *@
+                            <AddToQueueButton Track="@track" />
                             <SharePopover EntryKey="@track.EntryKey" />
                         </div>
                     }

DeepDrftPublic.Client/Pages/MixDetail.razor

@@ -81,6 +81,8 @@ else
                         @if (ViewModel.Track is not null)
                         {
                             <PlayStateIcon Track="@ViewModel.Track" Size="Size.Large" Color="Color.Secondary" OnToggle="@PlayTrack" />
+                            @* Append-only: queues the mix's single track without starting playback. *@
+                            <AddToQueueButton Track="@ViewModel.Track" Size="Size.Large" />
                         }
                     </PlayContent>
                 </ReleaseHeroOverlay>

DeepDrftPublic.Client/Pages/SessionDetail.razor

@@ -79,6 +79,8 @@ else
                 @if (ViewModel.Track is not null)
                 {
                     <PlayStateIcon Track="@ViewModel.Track" Size="Size.Large" Color="Color.Secondary" OnToggle="@PlayTrack" />
+                    @* Append-only: queues the session's single track without starting playback. *@
+                    <AddToQueueButton Track="@ViewModel.Track" Size="Size.Large" />
                 }
             </PlayContent>
         </ReleaseHeroOverlay>