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/storage.ts

@@ -421,6 +421,8 @@ export function normalizeSettings(settings: AppSettings): AppSettings {
     packageName: asText(settings.packageName),
     autoExtract: Boolean(settings.autoExtract),
     autoRename4sf4sj: Boolean(settings.autoRename4sf4sj),
+    keepGermanAudioOnly: Boolean(settings.keepGermanAudioOnly),
+    germanAudioMode: settings.germanAudioMode === "first" ? "first" : "tag",
     extractDir: normalizeAbsoluteDir(settings.extractDir, defaults.extractDir),
     collectMkvToLibrary: Boolean(settings.collectMkvToLibrary),
     mkvLibraryDir: normalizeAbsoluteDir(settings.mkvLibraryDir, defaults.mkvLibraryDir),