feat(queue): two-level deque model — PLAY prepends, add appends, last-track-end empties

Fixes five queue bugs: Playlist relabel, last-track-empties, dormant-seed-from-player on first add, immediate panel reactivity, and front/back deque semantics. Adds JumpTo for row jumps.

DeepDrftPublic.Client/Controls/AddToQueueButton.razor

@@ -3,10 +3,11 @@
 @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. *@
+   cascaded IQueueService's Enqueue/EnqueueRange (which append to the END without disturbing current
+   playback; a first add into a dormant queue seeds the head from the externally-playing track when one
+   exists, then appends) — never PlayRelease/PlayTrack/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"