fix: CD metadata dialog search/images/Weiter button

- Remove auto MusicBrainz search from analyzeCd; user triggers search manually
- Dialog uses single results state (replaced per search, not appended)
- Add cover-art-archive to MB search includes so cover images load
- Weiter button only blocked by track selection when tracks actually exist

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

backend/src/services/musicBrainzService.js

@@ -92,7 +92,7 @@ class MusicBrainzService {
     url.searchParams.set('query', q);
     url.searchParams.set('fmt', 'json');
     url.searchParams.set('limit', '10');
-    url.searchParams.set('inc', 'artist-credits+labels+recordings');
+    url.searchParams.set('inc', 'artist-credits+labels+recordings+cover-art-archive');
 
     try {
       const data = await mbFetch(url.toString());