feat(mpd): add MPD extension

[psic4t]

Summary

Adds an MPD extension that lets you browse and control a running
Music Player Daemon server from Vicinae.

Commands

• Show Play Queue — current queue with album covers; jump to or remove songs.
• Show Albums — all albums sorted by last added; play or append an album to the queue.
• Search Songs — search the library by title, artist, or album.

Requirements

A running MPD server reachable over TCP (MPD_HOST / MPD_PORT, defaults
localhost:6600) or a Unix socket (absolute path in MPD_HOST). No extra
CLI tools — the extension talks to MPD directly via the bundled mpc-js
client.

Album art is fetched via readpicture with an albumart fallback and
cached locally under ~/.cache/vicinae-mpd/art.

Webradio support

The play queue handles webradio streams as well as local files:

• The currently-playing track (from ICY metadata) is shown on top with the
  station name underneath; ICY title changes are picked up automatically
  while a stream is playing.
• Station icons are resolved from several sources in priority order:
  embedded stream art (MPD's readpicture) first, then the curated
  WebradioDB image set (by
  stream URL), then the radio-browser.info
  favicon, falling back to a generic stream glyph. Every downloaded image is
  validated (magic-byte sniffing + minimum size) so broken, empty, or
  HTML-error responses are never cached or displayed.
• Station enrichment: stream rows that arrive without a Name tag (e.g.
  added with bare mpc add http://…) are enriched with the station name and
  logo from radio-browser.info, looked up by stream URL with a name-based
  fallback. Results are cached locally (positive: 7 days; negative: 1 day).
  The mirror can be pinned with RADIO_BROWSER_API.

Privacy

Only your queue's stream URLs and host identities are sent to third parties
(radio-browser, and GitHub Pages for WebradioDB); when an exact URL match
fails, the station Name tag is also sent to radio-browser's search. No
other MPD data (your library, tags, or listening history) ever leaves your
machine.

Validation

• npx vici lint — manifest valid
• npm test — 235/235 passing
• npm run typecheck — clean
• npm run build — succeeds

Notes

• Category: Media.
• Icon at assets/extension_icon.png (512×512).
• License: MIT.