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).
Update test expectations to match the new _subtitle_lang_list() output
which includes regional variants (e.g. fr-FR, en-GB) for better
subtitle matching.
