feat(player): docked queue overlay with reorder, remove, jump, and clear-upcoming

Add a Queue toggle to the docked player bar opening a centered editable queue
overlay. New additive QueueService.ClearUpcoming keeps the playing track while
dropping the rest. Current track is non-removable.

DeepDrftPublic.Client/Controls/AudioPlayerBar/PlayerTransportZone.razor

@@ -20,5 +20,20 @@
                                  Indeterminate="@(LoadProgress == 0)"/>
         }
     </MudStack>
+    @* Queue toggle: a second row between the transport controls and the timestamp (§3.1 placement —
+       "below the control buttons, to the left of the timestamps"). Shown only when a queue is loaded,
+       mirroring the skip-affordance gating, so an empty/single-track player is byte-for-byte unchanged. *@
+    @if (ShowQueueButton)
+    {
+        <MudTooltip Text="Queue">
+            <MudIconButton Icon="@Icons.Material.Filled.QueueMusic"
+                           Color="Color.Primary"
+                           Size="Size.Medium"
+                           OnClick="QueueToggle"
+                           aria-label="Queue"
+                           aria-expanded="@QueueOpen"
+                           Class="@($"deepdrft-queue-toggle{(QueueOpen ? " deepdrft-queue-toggle-active" : "")}")"/>
+        </MudTooltip>
+    }
     <TimestampLabel CurrentTime="DisplayTime" Duration="@Duration"/>
 </MudStack>