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>

styles.css

@@ -775,6 +775,7 @@ button:focus{outline:none}
 .list-search{width:100%;box-sizing:border-box;padding:8px 12px 8px 32px;background:var(--surface);border:1px solid var(--border2);border-radius:8px;color:var(--text1);font-size:13px;font-family:var(--font);outline:none;transition:border-color .15s}
 .list-search:focus{border-color:var(--accent)}
 .list-search::placeholder{color:var(--text3)}
+.importer-section-label{display:flex;align-items:center;gap:7px;font-size:11px;font-weight:700;text-transform:uppercase;letter-spacing:1.2px;margin-bottom:10px}
 #spotify-enrich-status{display:flex;align-items:center;gap:8px;font-size:12px;color:var(--text2);padding:8px 12px;margin-bottom:8px;background:var(--surface);border:1px solid var(--border2);border-radius:8px;transition:opacity .4s}
 #spotify-enrich-status.done{color:var(--accent)}
 #spotify-enrich-status .enrich-spinner{width:12px;height:12px;border:2px solid var(--border2);border-top-color:var(--accent);border-radius:50%;animation:spin .7s linear infinite;flex-shrink:0}