fix: enable player controls on load, clear track selection on stop and end-of-track

Add StateChanged multicast event to IPlayerService so AudioPlayerBar and TracksView
re-render themselves without relying on the IsFixed cascade re-render path. Clear
_selectedTrack in TracksView when IsLoaded drops (stop, unload, end-of-track). Set
IsLoaded=false in OnPlaybackEndCallback so end-of-track triggers the same clear path.
Add JS-module readiness probe in AudioInteropService; delete dead TS and buffered C#
path; consolidate GetPlayIcon/FormatTime helpers; fix misleading minimize dock icon.

DeepDrftPublic.Client/Controls/AudioPlayerBar/AudioPlayerBar.razor

@@ -2,8 +2,8 @@
 {
     <div class="minimized-dock d-flex align-center justify-center"
          @onclick="@ToggleMinimized">
-        <MudIconButton Icon="@GetPlayIcon()"
-                       Color="Color.Primary" 
+        <MudIconButton Icon="@Icons.Material.Filled.ExpandLess"
+                       Color="Color.Primary"
                        Size="Size.Large"
                        Class="minimized-button"
                        OnClick="@ToggleMinimized"/>