[bluebird42]

What

Core can now publish audio descriptions (maveio/core#39): per language a full-mix MP3 with spoken descriptions, as an HLS alternate audio rendition marked CHARACTERISTICS="public.accessibility.describes-video", and a descriptions WebVTT, listed in a new manifest key:

"audio_descriptions": [
  {"language": "nl", "label": "Dutch (audio description)",
   "audio": {"path": "…/audio_description_nl.mp3", "hls_playlist": "audio_description_nl_hls/playlist.m3u8"},
   "vtt": {"path": "…/descriptions_nl.vtt"}}
]

This PR lets mave-player play them:

• AD button (src/components/audio-description-button.ts) in the control bar, with aria-pressed. On switches to the describes-video audio track in the current audio language (falling back to video.language, then en); off goes back to the previous track. The choice is remembered per viewer.
• audiodescription attribute: auto (default: shown when the manifest has audio descriptions and playback is HLS), on, off. Not shown for MP4-only playback.
• Descriptions track: each VTT is added as a hidden <track kind="descriptions">, for assistive technology, and stays out of the captions menu.
• Placement in the default and dolphin themes; strings localized.
• Embed.audio_descriptions type in src/embed/api.ts.
• Example: examples/audio-description.html with a small local HLS fixture.

Manifests without audio_descriptions behave exactly as before.

Verified

• npm run ts-types and npm run build pass. npm run lint reports no new errors (the changed files had the same count before).
• In the browser with HLS.js: the button shows, toggling switches between the original and the describes-video track both ways, and the descriptions track is present but not in the captions menu.
• Not yet tested with Safari's native HLS.