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/main.js

@@ -20,6 +20,7 @@ import {
 } from './views.js';
 import { loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
          onImporterInput, resolveImporterLink } from './spotify.js';
+import { resolveYouTubeLink, executeYouTubeDownload } from './youtube.js';
 import { loadSoundCloudImporter, resolveSoundCloudLink, clearSoundCloudPreview, scDlAll, scDlSingle, scDlTrack } from './soundcloud.js';
 import {
   absInitFromStorage, absOpenSettings, absConnectSettings, absSetLibrary, absDisconnect, absVerifyApiKey,
@@ -311,7 +312,7 @@ Object.assign(window, {
   loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll,
   onImporterInput, resolveImporterLink,
   loadSoundCloudImporter, resolveSoundCloudLink, clearSoundCloudPreview, scDlAll, scDlSingle, scDlTrack,
-  executeSpotifyDownload,
+  executeSpotifyDownload, resolveYouTubeLink, executeYouTubeDownload,
   openPlModal, closePlModal, addSongsToPlaylist, createAndAddPlaylist, createPlaylistPrompt,
   showSongInfo, closeInfoModal, toggleInfoEdit, onInfoArtFile, autofillMusicBrainz, saveMetadata,
   openDlModal, closeDlModal, selectDlType, executeDownload,