feat: Deezer artist photo fallback for local artists with no Navidrome image

Niche local artists (e.g. N!GHT) whose Navidrome metadata agents have no image showed a
blank hero. Now, when Navidrome has no artist image, the hero falls back to the matched
Deezer artist's photo (Navidrome image → Deezer photo → album cover), on web + Android.

- Web (detail.js): the Deezer-photo fallback previously ran only for non-local artists;
  now it runs whenever the Navidrome image is absent, local or not.
- Android: ArtistViewModel captures the matched Deezer artist's photo (deezerImage);
  ArtistDetailScreen uses it as the hero fallback.
SW ns-v56.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v55';
+const CACHE = 'ns-v56';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.