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/Services/QueueService.cs

@@ -201,6 +201,20 @@ public sealed class QueueService : IQueueService, IDisposable
         QueueChanged?.Invoke();
     }
 
+    public void ClearUpcoming()
+    {
+        // Keep the currently-playing track, drop everything else. No current track (dormant/empty) or a
+        // queue that already holds only the current → nothing to clear.
+        var current = Current;
+        if (current is null || _items.Count <= 1) return;
+
+        _items.Clear();
+        _items.Add(current);
+        CurrentIndex = 0;
+        // Playback is untouched (C2): the current track keeps streaming; we only discarded the up-next.
+        QueueChanged?.Invoke();
+    }
+
     // Advance on organic end-of-stream only. TrackEnded is not raised by stop/unload/track-switch,
     // so a manual stop or a fresh single-track selection elsewhere never spuriously advances the
     // queue. When the queue is past its last track, end-of-stream simply stops — nothing to advance.