fix(artist): show fanart in the artist-detail hero, not the Navidrome cover (#1172)

* fix(cover): report a genuine miss for external artist surfaces

When external artwork is enabled, the fanart/banner ensure fell through to
the Navidrome cover and returned it as a hit on a fanart.tv miss. That made
'this artist has no banner' indistinguishable from a real hit, so the
artist-detail hero short-circuited on the ND cover and never reached the
fanart it actually had. Return hit=false on an external-surface miss instead;
the Navidrome fallback is the caller's job, since each surface has its own
chain (hero: banner->fanart->ND, fullscreen player: fanart->ND).

* fix(artist): hero background banner->fanart->Navidrome with raised focal point

The hero header background now steps through banner -> fanart -> Navidrome
artist cover, resolving the ND cover the same way the fullscreen player does.
Previously it had no Navidrome stage at all and, combined with the backend
masking a banner miss as a hit, showed the ND cover where the fanart belonged.

Also raise the focal point (background-position: center 30%) for the portrait-ish
fanart/ND images so the band's heads stay in frame on wide (2K+) viewports,
where 'cover' scales them up and overflows vertically. The wide banner strip
keeps the shared center. Done via a scoped inline style; .album-detail-bg stays
untouched for the album/playlist headers that share it.

* docs(changelog): artist header external-background fallback fix (#1172)

CHANGELOG.md

@@ -120,6 +120,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Fixed
 
+### Artist header showing the plain image instead of the external background
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1172](https://github.com/Psychotoxical/psysonic/pull/1172)**
+
+* On the artist page, when an artist had an external background image (from fanart.tv) but no banner, the header showed the plain Navidrome artist image instead of the background — even though the fullscreen player used the background correctly. The header now falls back banner → background → Navidrome image as intended. The background also sits a little higher so band members' heads aren't cropped on wide screens.
+
 ### Playlists header buttons clipped at narrow widths
 
 **By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1153](https://github.com/Psychotoxical/psysonic/pull/1153)**