feat: unified artist page with full iTunes/Navidrome album categorization

- loadArtistDetail now works for both Navidrome artists and iTunes-only
  artists (id=null + itunesArtistId). loadItunesArtistResults from global
  search now navigates to the same artist-detail page.
- Album sections: "In Library" (fully downloaded), "Unvollständig" (partial
  — e.g. 2/12 tracks), "Noch nicht heruntergeladen" (iTunes-only). Partial
  albums show X/Y badge and a direct download button on hover.
- iTunes albums now fetched via lookup endpoint (up to 200 albums) instead
  of search (max 20), fixing the missing-albums problem.
- "Alle X Songs in Library" expand button fetches all songs across albums
  and renders them grouped by album with play/context-menu support.
- Back button uses _navGoBack() for context-aware navigation.
- CSS: .section-count, .album-partial-badge, .card-dl-btn (hover reveal)

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

index.html

@@ -174,8 +174,7 @@
         <div id="artist-bio-section"></div>
         <div id="artist-similar-section"></div>
         <div id="artist-topsongs-section"></div>
-        <div class="section-title" style="margin-bottom:12px;margin-top:8px">Alben</div>
-        <div id="artist-albums-grid" class="grid grid-4"></div>
+        <div id="artist-albums-section"></div>
       </div>
 
       <!-- ALBUMS -->