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.

Views/ProgressWindow.xaml.cs

@@ -174,7 +174,7 @@ public partial class ProgressWindow : FluentWindow
                         .Where(t => t.Type == "subtitles" && t.Copy)
                         .Select(t => t.Id).ToList();
 
-                    bool nothingToStrip = file.Tracks.All(t => t.Copy);
+                    bool nothingToStrip = file.Tracks.All(t => t.Copy) && file.ExternalSubtitles.Count == 0;
 
                     var progress = new Progress<ProcessingProgress>(p =>
                     {
@@ -187,10 +187,14 @@ public partial class ProgressWindow : FluentWindow
                         : await service.StripTracksAsync(
                             file.FilePath, outputFilePath,
                             videoIds, audioIds, subIds,
+                            file.ExternalSubtitles.ToList(),
                             progress, ct);
 
                     string tag = result.Success ? "[DONE]" : "[FAIL]";
-                    AppendLog($"{tag} {Path.GetFileName(result.OutputFilePath)} — {result.Message}");
+                    string extInfo = file.ExternalSubtitles.Count > 0
+                        ? $" (+{file.ExternalSubtitles.Count} external sub{(file.ExternalSubtitles.Count == 1 ? "" : "s")})"
+                        : string.Empty;
+                    AppendLog($"{tag} {Path.GetFileName(result.OutputFilePath)}{extInfo} — {result.Message}");
 
                     HistoryService.Add(new HistoryEntry
                     {