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/spotify.js

@@ -10,6 +10,7 @@ export function onImporterInput(val) {
   // Always clear results when URL changes
   document.getElementById('spotify-preview').style.display = 'none';
   document.getElementById('sc-preview').style.display = 'none';
+  document.getElementById('yt-preview').style.display = 'none';
   document.getElementById('importer-hint').style.display = val ? 'none' : '';
 
   const badge = document.getElementById('importer-badge');
@@ -29,6 +30,13 @@ export function onImporterInput(val) {
     btn.style.background = '#ff5500';
     btn.style.color = '#fff';
     btn.style.borderColor = 'transparent';
+  } else if (val.includes('youtube.com') || val.includes('youtu.be')) {
+    badge.textContent = 'YouTube';
+    badge.style.background = 'rgba(255,0,0,.15)';
+    badge.style.color = '#FF0000';
+    btn.style.background = '#FF0000';
+    btn.style.color = '#fff';
+    btn.style.borderColor = 'transparent';
   } else {
     badge.textContent = 'URL';
     badge.style.background = '';
@@ -48,6 +56,8 @@ export async function resolveImporterLink() {
   document.getElementById('importer-hint').style.display = 'none';
   if (url.includes('soundcloud.com')) {
     window.resolveSoundCloudLink();
+  } else if (url.includes('youtube.com') || url.includes('youtu.be')) {
+    window.resolveYouTubeLink();
   } else {
     resolveSpotifyLink();
   }