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).
Two fixes for captions downloads:
1. Language variant fallback: When user requests 'en' but the video only
has 'en-GB' (or other regional variants), yt-dlp silently skips
subtitle extraction because it uses exact language matching. Now the
subtitleslangs list includes common regional variants (e.g. en-US,
en-GB, en-AU, etc.) so yt-dlp can match the first available variant.
2. Error on no files: When yt-dlp completes successfully (ret=0) but
produces no subtitle files (e.g. no matching language at all), the
download was incorrectly shown as 'completed'. Now it's marked as
'error' with a descriptive message like 'No subtitles found for
language "en"'. Same treatment for thumbnail downloads.
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.
The iOS-compatible video may not be the best quality. Add a separate quality option to accommodate people who want the best available versus the best compatible with iOS's strict requirements.
Testing with https://www.youtube.com/watch?v=YiRMs5ZhcH4 where the best quality video is 2160p and not iOS-compatible.
With best quality, the VP9 video format is used (better quality but not iOS-compatible):
```
% ffprobe -hide_banner Who\ Can\ Find\ the\ Weirdest\ PC\ Parts\ on\ AliExpress？.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Who Can Find the Weirdest PC Parts on AliExpress？.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2mp41
encoder : Lavf60.16.100
Duration: 00:19:02.72, start: 0.000000, bitrate: 10941 kb/s
Stream #0:0[0x1](und): Video: vp9 (Profile 0) (vp09 / 0x39307076), yuv420p(tv, bt709), 3840x1920, 10805 kb/s, 29.97 fps, 29.97 tbr, 16k tbn (default)
Metadata:
handler_name : ISO Media file produced by Google Inc. Created on: 06/15/2024.
vendor_id : [0][0][0][0]
Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : ISO Media file produced by Google Inc.
vendor_id : [0][0][0][0]
```
With "Best (iOS)" quality, the H264 video (lower quality but iOS-compatible) is used:
```
% ffprobe -hide_banner Who\ Can\ Find\ the\ Weirdest\ PC\ Parts\ on\ AliExpress？.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Who Can Find the Weirdest PC Parts on AliExpress？.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf60.16.100
Duration: 00:19:02.72, start: 0.000000, bitrate: 1846 kb/s
Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 1920x960 [SAR 1:1 DAR 2:1], 1710 kb/s, 29.97 fps, 29.97 tbr, 30k tbn (default)
Metadata:
handler_name : ISO Media file produced by Google Inc.
vendor_id : [0][0][0][0]
Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : ISO Media file produced by Google Inc.
vendor_id : [0][0][0][0]
```
Included a README note about the new quality option.
iOS has strict requirements for video files, requiring h264 or h265 video codec and aac audio codec in MP4 container. This update to the MP4 format string tries to get a fully compatible file first, followed by the right video codec and any M4A audio (audio is much faster to convert if needed), and then falls back to the original behaviour of getting the best available MP4 video and M4A audio.
