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/main.js

@@ -42,6 +42,7 @@ import {
   toggleMobileSearch, closeMobileSearch, switchSetupTab,
   toggleNotifPanel, deleteNotif, clearNotifs, initNotifBell, _notifNavigate,
   _pickTheme,
+  toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, saveMetadata,
 } from './ui.js';
 
 // ─── CONNECT ───
@@ -308,7 +309,7 @@ Object.assign(window, {
   loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
   executeSpotifyDownload,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist, createPlaylistPrompt,
-  showSongInfo, closeInfoModal,
+  showSongInfo, closeInfoModal, toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, saveMetadata,
   openDlModal, closeDlModal, selectDlType, executeDownload,
   openEditPlModal, closeEditPlModal, saveEditPlaylist,
   editPlUploadCover, editPlCoverSelected, editPlRemoveCover, deletePlaylist, _confirmOk, _confirmCancel,