feat: SoundCloud downloader + rename page to Song Importer

- New src/soundcloud.js: resolves SoundCloud URLs via yt-dlp --dump-single-json,
  renders track/playlist preview with per-track download buttons
- Backend POST /api/soundcloud/resolve: extracts metadata (title, artist, genre,
  artwork, duration) directly from SoundCloud — no iTunes lookup needed
- ytdlp.js: sourceUrl param — if set, downloads directly from URL instead of
  ytsearch cascade (used for SoundCloud tracks)
- Spotify importer page renamed to Song Importer, now contains both
  Spotify and SoundCloud sections in one view

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

src/main.js

@@ -19,6 +19,7 @@ import {
   plReorderEnter, plReorderSave, plReorderCancel, toggleAllGenres,
 } from './views.js';
 import { loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll } from './spotify.js';
+import { loadSoundCloudImporter, resolveSoundCloudLink, clearSoundCloudPreview, scDlAll } from './soundcloud.js';
 import {
   absInitFromStorage, absOpenSettings, absConnectSettings, absSetLibrary, absDisconnect, absVerifyApiKey,
   absPlayItem, absTogglePlay, absSeek, absSkipSecs, absNextChapter, absPrevChapter,
@@ -307,6 +308,7 @@ Object.assign(window, {
   loadAlbums, loadStarred, loadRandom, filterStarred, filterPlaylist,
   plReorderEnter, plReorderSave, plReorderCancel, toggleAllGenres,
   loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
+  loadSoundCloudImporter, resolveSoundCloudLink, clearSoundCloudPreview, scDlAll, scDlSingle, scDlTrack,
   executeSpotifyDownload,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist, createPlaylistPrompt,
   showSongInfo, closeInfoModal, toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, saveMetadata,