feat: YouTube/YT Music importer + album artist in view mode

- Info modal view now shows Album-Künstler row from Navidrome albumArtist
- Song Importer detects youtube.com / youtu.be URLs (red badge)
- New /api/youtube/resolve endpoint: yt-dlp --dump-json for single
  videos and playlists (up to 50 entries), enriched with iTunes lookup
  for proper album/year/genre/cover on single tracks
- New src/youtube.js renders preview card and delegates download to
  executeSpotifyDownload (which already handles sourceUrl → direct yt-dlp)
- Importer hint updated to list YouTube as a supported source

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

src/ui.js

@@ -243,9 +243,10 @@ function _renderInfoModal(s) {
   document.getElementById('info-top-title').textContent = s.title || '—';
   document.getElementById('info-top-sub').textContent = (s.artist||'—') + (s.album ? ' · '+s.album : '');
   const fields = [
-    ['Titel',       s.title],
-    ['Künstler',    s.artist],
-    ['Album',       s.album],
+    ['Titel',          s.title],
+    ['Künstler',       s.artist],
+    ['Album-Künstler', s.albumArtist],
+    ['Album',          s.album],
     ['Jahr',        s.year],
     ['Track #',     s.track],
     ['Disc #',      s.discNumber],