Add "keep only German audio" post-extract step for .DL. files

- New video-processor.ts: ffmpeg/ffprobe remux that keeps only the German
  audio track (by language tag, with safe fallbacks) and strips the ".DL."
  marker from the filename
- Runs after extraction in both the deferred and hybrid post-process paths,
  inside the per-package file-op chain; abortable, disk-space checked,
  mtime-preserving, atomic temp->replace so the original is never lost
- System ffmpeg via PATH / RD_FFMPEG_BIN; toggle + track-mode select in settings

src/shared/types.ts

@@ -97,6 +97,8 @@ export interface AppSettings {
   packageName: string;
   autoExtract: boolean;
   autoRename4sf4sj: boolean;
+  keepGermanAudioOnly: boolean;
+  germanAudioMode: "tag" | "first";
   extractDir: string;
   collectMkvToLibrary: boolean;
   mkvLibraryDir: string;