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/views.js

@@ -8,6 +8,7 @@ import { closeNowPlaying, closeNpMenu } from './nowplaying.js';
 import { makeCard, makeAlbumCard, makeSongItem, makeGlobalTrackItem, mkSection } from './components.js';
 import { fetchRomajiIndex } from './api.js';
 import { loadSpotifyImporter } from './spotify.js';
+import { loadSoundCloudImporter } from './soundcloud.js';
 
 // ─── NAVIGATION HISTORY ───
 export function _navSave() {
@@ -52,7 +53,7 @@ export function navigate(page, data=null) {
   if(page==='playlists')     loadPlaylists();
   if(page==='playlist-detail' && data) loadPlaylistDetail(data);
   if(page==='recommendations')  loadRecommendations();
-  if(page==='spotify-importer') loadSpotifyImporter();
+  if(page==='spotify-importer') { loadSpotifyImporter(); loadSoundCloudImporter(); }
   if(page==='audiobooks') {
     import('./abs.js').then(m => {
       if (S.abs.currentLibraryId) m.loadAbsBooks(S.abs.currentLibraryId);