feat(hero): album-artist backdrop, configurable per-surface sources, and prefetch (#1193)

* refactor(cover): extract shared pickArtistBackdrop priority helper

* feat(hero): show the album artist's fanart as the mainstage hero backdrop

* feat(settings): configurable per-surface artist backdrop sources

Each artist-backdrop surface (mainstage hero, artist-detail header, fullscreen player) gets its own enable toggle + an ordered, individually-toggleable source list, configured under External Artwork Scraper on the Integrations tab (shown when the scraper is on). Reorder via the shared useDragSource/psy-drop drag infra plus keyboard-accessible up/down buttons; each source has its own on/off.

The shared chooser pickArtistBackdrop is generalised to resolveBackdrop + backdropFromConfig (ordered candidate list with the same pending/miss/centred-framing semantics), so all three surfaces resolve identically. themeStore persist bumped to v2; defaults reproduce today's order, so there is no visible change without user action.

Gating decoupled: the three surfaces are gated solely by their own per-surface flag, not by enableCoverArtBackground (which stays scoped to album/playlist-header cover blur). Reorder maths extracted to a pure, unit-tested module. i18n en + de (other locales TODO before PR).

Tests: resolver (10) + reorder (7).

* feat(cover): make the ensure queue surface-aware for artist backdrops

coverEnsureQueued now threads optional CoverEnsureOpts through to the Rust ensure and weaves the external surface into the in-flight key, so the fanart and banner surfaces of one artist no longer collide on one download chain. External surfaces also bypass the disk-src memory short-circuit (their {tier}-{surface}.webp never seeds those caches, and the canonical cover must not read as a hit). New thin ensureArtistBackdropQueued wrapper. Backward-compatible: plain covers append nothing to the key; the 5 queue tests stay green.

* fix(cover): reset the artist external-image hook synchronously on artist change

The hook reset src in an effect (one render late), so for the render between an artist change and that effect a consumer read the *previous* artist's resolved image. The mainstage hero then froze (and cached into per-album memory) a neighbouring slide's banner. Reset synchronously via the React adjust-state-on-prop-change pattern. Also removes brief stale-image flashes on the artist-detail header and fullscreen player.

* feat(hero): prefetch artist backdrops and show-ready-now / upgrade-on-re-entry

warmHomeMainstageCovers now prefetches each hero slide's artist backdrop (banner/fanart) at static slide-index priorities (idx1=high, idx0=low, rest=middle; no reprioritise on navigation), then predecodes every slide already on disk. useHeroBackdrop shows the best source ready at entry (Navidrome on a cold first visit, the prefetched/cached external one on re-entry) and freezes that source choice for the visit so nothing swaps mid-dwell; the url is derived live from the frozen choice, with a per-album disk memory for re-entry. HeroBg now crossfades only after the image bytes load (onLoad/complete gate + onError + fallback). Inert when the scraper is off. Tests: per-album memory (5).

* docs(changelog): configurable artist backdrops + mainstage hero (PR #1193)

CHANGELOG Added entry, settingsCredits line, and the What's New Artist-artwork highlight extended to cover the mainstage hero backdrop and per-place source config.

* docs(changelog): fold mainstage hero + per-place backdrops into the fanart entry (PR #1193)

Merge the configurable-backdrops changes into the existing 'Artist artwork from fanart.tv' block (now PR #1137 and #1193) instead of a separate entry.

* fix(hero): revert HeroBg load-gate that blanked the app (Maximum update depth)

The byte-load gate I added drove the crossfade reveal from an inline img ref + onLoad that re-fires on every render and scheduled a setTimeout each call; frequent re-renders (playback, marquee) stacked nested updates until React threw 'Maximum update depth exceeded' — and with no ErrorBoundary the whole window blanked. Reverted HeroBg to the proven timer-based crossfade. The hero only ever receives ready/predecoded urls, so the gate was cosmetic.

* fix(hero): gate the HeroBg crossfade on image load to stop the slide flicker

The bare 20 ms reveal faded a layer in before its bytes were ready (notably the Navidrome raw url), so a slide change flickered. Now an Image() preloader in the [url] effect reveals the layer on load (or a cached complete check), with a fallback. Everything is scheduled once per url — no per-render <img> ref/onLoad — so unlike the reverted gate it can't stack nested updates.

* feat(i18n): translate the backdrop-source settings into the remaining 10 locales

Adds the per-surface backdrop config strings (es, fr, nl, zh, nb, ru, ro, ja, hu, pl) and extends externalArtworkDesc to mention the mainstage hero where the key exists (ja has none → falls back to en).

CHANGELOG.md

@@ -68,9 +68,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Artist artwork from fanart.tv
 
-**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1137](https://github.com/Psychotoxical/psysonic/pull/1137)**
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1137](https://github.com/Psychotoxical/psysonic/pull/1137) and PR [#1193](https://github.com/Psychotoxical/psysonic/pull/1193)**
 
 * New opt-in **External Artwork Scraper** (Settings → Integrations, off by default): artist imagery from fanart.tv — a 16:9 background on the fullscreen player and a wide banner on the artist page — with Navidrome staying the canonical cover. Optional personal key; turning it off removes the fetched images again.
+* The **mainstage hero** on the home screen now shows the album artist's backdrop too, matching the fullscreen player and artist page.
+* Choose, per place (mainstage hero, artist page, fullscreen player), which images to use as the background and in what order — drag to reorder or switch a source off, under the same setting. The hero also preloads the upcoming backdrops so they appear without a long blank.
 
 ### Remember the equalizer per audio output device