fix(transcoding): audio selection for direct files without native track support

[shethshlok]

Description

The startup probe skips the transmux pipeline when a direct MP4 or MKV file's default audio already plays. In browsers without HTMLMediaElement.audioTracks, this leaves the player with no track list. The Audio menu stays hidden even when the file contains multiple playable audio tracks.

Keep the existing transmux pipeline for these multi-track files when the browser lacks native track selection. The existing Audio menu can then switch tracks. Single-track files and browsers with native track selection keep their current playback path. HLS playback is unchanged.

This uses the existing codec and source-access checks. Files that the browser cannot read through CORS, or whose codecs the pipeline cannot handle, still cannot expose alternate tracks.

Type of change

• Bug fix
• Tests

Related issues

Fixes #23. The issue includes reproduction steps, a generated-media fixture and regression-test logs.

Validation

• The new missing-selector test fails with upstream's original transmux-session.ts and passes with this fix. Issue fix: Audio menu missing for multi-track direct files without native track support #23 includes the failure output.
• bun run check passed with no errors. The service-worker check still reports the existing warning about having no Svelte input files.
• All 23 focused player tests passed. The new tests cover missing native track support, native track support, single-track playback and preferred audio language.
• Biome passed for all four changed files.
• Tested the actual player in Chromium with a generated H.264/AAC file containing English and Hindi audio tracks. Both menu options appeared, and playback continued when switching in both directions. Captured audio changed between the fixture's 440 Hz and 880 Hz tones.

Screenshots

No screenshots attached. The playback check above verified both the menu selection and the resulting audio.

Checklist

• My code follows the project's code style.
• I have reviewed the changes.
• I have added tests for the changed behavior.
• I have updated the affected code comments.
• My changes introduce no new warnings or errors in the checks above.

[orochibraru]

Hey, I'm looking into this.
Realising that my CI setup isn't fork-friendly, I'll fix that asap do E2E can run.

[shethshlok]

Thanks!

[orochibraru]

Aside from a few comments, LGTM!
Thank you for submitting a PR.

[shethshlok]

I've pushed the updates based on your recommendations. The test import is now at the top of the file, and the audio fallback condition uses separate checks. Biome, type checks and all 918 unit tests passed locally.

Thank you for reviewing my PR and welcoming my contribution to the project. I've been using it actively over the past week, and I'd love to contribute more. If I find any issues, I'll report them and work on PRs to fix them.