Match and mux in dropped external subtitles

Drag in subtitle files alongside .mkv files and they're matched to
the right video by episode code (S04E06, zero-padding and separator
insensitive, plus 4x06 as a fallback) and muxed in as extra subtitle
tracks. Any subtitle format mkvmerge accepts is recognized: SRT,
(S)SA, VobSub, WebVTT, PGS/SUP, USF, SAMI, TTML/DFXP, STL, Kate.

Matching works regardless of drop order — subtitles dropped before
their video are held pending until a matching video shows up, and
vice versa. Language, forced, and SDH flags are guessed from common
filename tokens (e.g. "eng", "forced", "sdh") to set the muxed
track's language/name/flags; unmatched subtitles are reported in the
status bar.

Models/QueuedFileItem.cs

@@ -43,6 +43,19 @@ public class QueuedFileItem : INotifyPropertyChanged
     public string FileSizeDisplay { get; }
     public ObservableCollection<TrackItem> Tracks { get; } = new();
 
+    /// <summary>
+    /// External subtitle files (dropped separately and matched to this file by
+    /// episode code, e.g. S04E06) that will be muxed in as additional
+    /// subtitle tracks when this file is processed.
+    /// </summary>
+    public ObservableCollection<ExternalSubtitleTrack> ExternalSubtitles { get; } = new();
+
+    public bool HasExternalSubtitles => ExternalSubtitles.Count > 0;
+
+    public string ExternalSubtitlesTooltip => ExternalSubtitles.Count == 0
+        ? string.Empty
+        : "External subtitles to mux in:\n" + string.Join("\n", ExternalSubtitles.Select(s => $"- {s.FileName} ({s.LanguageDisplay})"));
+
     public string Status
     {
         get => _status;
@@ -71,7 +84,12 @@ public class QueuedFileItem : INotifyPropertyChanged
             int audioSelected = Tracks.Count(t => t.Type == "audio" && t.Copy);
             int subtitleSelected = Tracks.Count(t => t.Type == "subtitles" && t.Copy);
 
-            return audioSelected == 1 && subtitleSelected == 1
+            // A subtitle requirement is met either by picking exactly one
+            // internal track, or by having at least one matched external
+            // subtitle file (which can also be combined with one internal track).
+            bool subtitleOk = subtitleSelected == 1 || (subtitleSelected == 0 && ExternalSubtitles.Count > 0);
+
+            return audioSelected == 1 && subtitleOk
                 ? FileLoadState.Correct
                 : FileLoadState.Incorrect;
         }
@@ -87,6 +105,7 @@ public class QueuedFileItem : INotifyPropertyChanged
         FilePath = filePath;
         FileSizeDisplay = FormatFileSize(filePath);
         Tracks.CollectionChanged += OnTracksChanged;
+        ExternalSubtitles.CollectionChanged += OnExternalSubtitlesChanged;
     }
 
     private static string FormatFileSize(string filePath)
@@ -112,6 +131,13 @@ public class QueuedFileItem : INotifyPropertyChanged
         }
     }
 
+    private void OnExternalSubtitlesChanged(object? sender, NotifyCollectionChangedEventArgs e)
+    {
+        OnPropertyChanged(nameof(HasExternalSubtitles));
+        OnPropertyChanged(nameof(ExternalSubtitlesTooltip));
+        NotifyLoadState();
+    }
+
     private void OnTracksChanged(object? sender, NotifyCollectionChangedEventArgs e)
     {
         if (e.NewItems != null)