Consolidate play/pause icon logic into PlaybackIcons mapper and PlayStateIcon component

Add Disabled parameter to PlayStateIcon; forward to MudIconButton;
pass Disabled="!IsLoaded" from PlayerControls to match Stop button parity.

DeepDrftPublic.Client/Helpers/PlaybackIcons.cs

@@ -0,0 +1,14 @@
+using MudBlazor;
+
+namespace DeepDrftPublic.Client.Helpers;
+
+/// <summary>
+/// Single source of truth for mapping playback state to a transport glyph across
+/// DeepDrftPublic.Client. Surfaces that render a play/pause icon call <see cref="Resolve"/>
+/// instead of inlining their own ternary.
+/// </summary>
+public static class PlaybackIcons
+{
+    public static string Resolve(bool isPlaying, bool isPaused)
+        => (isPlaying && !isPaused) ? Icons.Material.Filled.Pause : Icons.Material.Filled.PlayArrow;
+}