Add chapters display, gray out fixed tracks, spacebar preview

Models/TrackItem.cs

@@ -44,6 +44,20 @@ public class TrackItem : INotifyPropertyChanged
     /// </summary>
     public string DisplayName => Type == "subtitles" ? $"{Codec}/{Name}" : Name;
 
+    /// <summary>
+    /// Video is always kept, and chapters (a synthetic pseudo-track, not a
+    /// real mkvmerge track) are always kept too — neither is user-selectable,
+    /// so their Copy checkbox is shown checked but disabled.
+    /// </summary>
+    public bool IsEditable => Type is "audio" or "subtitles";
+
+    /// <summary>
+    /// Chapters aren't a real track and have no track ID; show a dash
+    /// instead of a meaningless number.
+    /// </summary>
+    public string IdDisplay => Type == "chapters" ? "—" : Id.ToString();
+
+
     public bool Copy
     {
         get => _copy;