feat: delete song from disk via edit-metadata modal

- /api/delete-song endpoint: resolves path via same fallback chain as
  edit-metadata, validates file is inside music root + is an audio file,
  deletes it, then prunes the album folder if empty and the artist folder
  if that too becomes empty — no recursive rm, depth strictly limited to
  two levels above the file so no accidental mass-deletes
- Safety: path.resolve() guard ensures no path-traversal outside music root;
  rmdirSync (non-recursive) fails if dir is non-empty as extra safety net
- Frontend: delete button (red outline, trash icon) at bottom of edit mode
  with confirmation dialog showing title + folder-cleanup warning

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

src/main.js

@@ -45,7 +45,7 @@ import {
   toggleMobileSearch, closeMobileSearch, switchSetupTab,
   toggleNotifPanel, deleteNotif, clearNotifs, initNotifBell, _notifNavigate,
   _pickTheme,
-  toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, autofillItunes, saveMetadata,
+  toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, autofillItunes, saveMetadata, deleteSong,
 } from './ui.js';
 
 // ─── CONNECT ───
@@ -316,7 +316,7 @@ Object.assign(window, {
   loadSoundCloudImporter, resolveSoundCloudLink, clearSoundCloudPreview, scDlAll, scDlSingle, scDlTrack,
   executeSpotifyDownload, resolveYouTubeLink, executeYouTubeDownload,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist, createPlaylistPrompt,
-  showSongInfo, closeInfoModal, toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, autofillItunes, saveMetadata,
+  showSongInfo, closeInfoModal, toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, autofillItunes, saveMetadata, deleteSong,
   openDlModal, closeDlModal, selectDlType, executeDownload,
   openEditPlModal, closeEditPlModal, saveEditPlaylist,
   editPlUploadCover, editPlCoverSelected, editPlRemoveCover, deletePlaylist, _confirmOk, _confirmCancel,