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/main/constants.ts

@@ -72,6 +72,8 @@ export function defaultSettings(): AppSettings {
     packageName: "",
     autoExtract: true,
     autoRename4sf4sj: false,
+    keepGermanAudioOnly: false,
+    germanAudioMode: "tag",
     extractDir: path.join(baseDir, "_entpackt"),
     collectMkvToLibrary: false,
     mkvLibraryDir: path.join(baseDir, "_mkv"),