P1 fixes:
- _subtitle_lang_list now extracts base language from qualified tags
(e.g. zh-Hans → zh base) so regional variants are still added as
fallbacks. Original input is preserved as first priority.
- DownloadInfo.__init__ now explicitly initializes filename=None and
chapter_files=[] to avoid AttributeError on dynamic attribute access.
- __setstate__ adds filename=None migration for old persistent data.
- Frontend Download interface gains subtitle_files field; completed
downloads show each subtitle file with download link and CC icon.
P2 improvements:
- _LANGUAGE_REGION_VARIANTS expanded from 10 to 30 languages covering
it/ja/ko/hi/th/vi/id/pl/uk/ru/cs/sv/da/no/fi/tr/el/he/hu/bn/ta.
- Languages with no regional variants use empty tuple (they still get
-orig suffix when include_orig=True).
Allow users to prefer a specific video codec (H.264, H.265, AV1, VP9)
when adding downloads. The selector filters available formats via
yt-dlp format strings, falling back to best available if the preferred
codec is not found. The completed downloads table now shows Quality
and Codec columns.
Display the completion time for each download in the done list.
The backend already stores a nanosecond timestamp on DownloadInfo; this wires it up to the frontend using Angular's DatePipe.
