feat: metadata editor with MusicBrainz autofill + cover art upload

- Info modal gets an edit pencil button that switches to edit form
- Editable fields: title, artist, album, genre, year, track#, disc#, total tracks
- Cover art: upload your own image or auto-fetch from Cover Art Archive via MusicBrainz
- MusicBrainz button queries /api/musicbrainz/lookup and autofills all fields
- Save POSTs to /api/edit-metadata which rewrites tags via ffmpeg and triggers Navidrome rescan
- Backend: GET /api/musicbrainz/lookup + POST /api/edit-metadata endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -32,6 +32,9 @@ export const S = {
   globalSearchTimer: null,
   _mobileSearchTimer: null,
 
+  // Song info / edit
+  infoSong: null,
+
   // Download
   downloadTarget: null,
   dlType: 'track',