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>

CHANGELOG.md

@@ -80,6 +80,7 @@ Big change: **the entire metadata/search stack moved from Apple's iTunes to Deez
 
 ### Fixed
 - **Web + Android** — Owned albums were wrongly shown as **"Unvollständig  1 / 0 Tracks"** on the artist page. Deezer's `/artist/{id}/albums` list omits `nb_tracks`, so the total read as 0 and every matched album looked incomplete. Web now resolves the real track count from the Deezer album endpoint for **matched (owned) albums only** and treats an unknown total as complete; Android treats an unknown total as complete (never a false partial)
+- **Web + Android** — Local Navidrome artist pages with **no artist image** (niche artists like N!GHT that Navidrome's metadata agents have no art for) showed a blank hero. They now fall back to the **matched Deezer artist photo** (Navidrome image → Deezer photo → album cover)
 
 ### Notes
 - Deploy: `git pull`, then rebuild the frontend (`docker compose up -d --build novasonic-frontend`) and restart the backend (`docker compose restart novasonic-backend`). Service worker bumped to `ns-v54`