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>

src/api.js

@@ -61,6 +61,12 @@ export async function itunesLookupArtistSongs(artistId, limit=15) {
   return (d.results || []).filter(x => x.wrapperType === 'track');
 }
 
+export async function itunesLookupArtistAlbums(artistId, limit=200) {
+  const r = await fetch(`${ITUNES}/lookup?id=${artistId}&entity=album&limit=${limit}&country=us`);
+  const d = await r.json();
+  return (d.results || []).filter(x => x.wrapperType === 'collection' && x.collectionType === 'Album');
+}
+
 export async function itunesTopCharts() {
   const urls = [
     'https://rss.applemarketingtools.com/api/v2/de/music/most-played/50/albums.json',