fix(fullscreen): preflight artist portrait URL so broken-img glyph never shows (#748)

Navidrome / Subsonic `getArtistInfo2` often returns a `largeImageUrl`
that 404s when the artist has no scraped image. The hook returned the
URL as-is, the FullscreenPlayer's `artistBgUrl || coverUrl` chain saw
a non-empty string and skipped the cover-art fallback, and FsPortrait
rendered an `<img>` whose `src` failed — leaving the browser's default
broken-image glyph in the middle of the screen.

Preflight the URL via an `Image()` probe and only expose it once it
actually loads; on error leave the hook's state empty so the cover-art
fallback in the caller takes effect.

src/hooks/useFsArtistPortrait.ts

@@ -3,17 +3,32 @@ import { getArtistInfo } from '../api/subsonicArtists';
 
 /** Fetches the large artist image for the given artist id, returning '' until
  *  the request resolves (or when there is no artist id). Falls through silently
- *  on network failures — the caller should layer a cover-art fallback on top. */
+ *  on network failures — the caller should layer a cover-art fallback on top.
+ *
+ *  Navidrome / Subsonic backends often return a `largeImageUrl` that 404s when
+ *  the artist has no scraped image (Last.fm placeholder), so the URL is
+ *  preflighted via an Image() probe and only exposed when it actually loads.
+ *  Otherwise the caller would render a broken-img glyph (`?`) on top of the
+ *  fullscreen background — see zunoz report on the Psysonic Discord. */
 export function useFsArtistPortrait(artistId: string | undefined): string {
   const [artistBgUrl, setArtistBgUrl] = useState<string>('');
   useEffect(() => {
     setArtistBgUrl('');
     if (!artistId) return;
     let cancelled = false;
+    let probe: HTMLImageElement | null = null;
     getArtistInfo(artistId).then(info => {
-      if (!cancelled && info.largeImageUrl) setArtistBgUrl(info.largeImageUrl);
+      if (cancelled || !info.largeImageUrl) return;
+      const url = info.largeImageUrl;
+      probe = new Image();
+      probe.onload = () => { if (!cancelled) setArtistBgUrl(url); };
+      probe.onerror = () => { /* leave empty so caller falls back to cover */ };
+      probe.src = url;
     }).catch(() => {});
-    return () => { cancelled = true; };
+    return () => {
+      cancelled = true;
+      if (probe) probe.onload = probe.onerror = null;
+    };
   }, [artistId]);
   return artistBgUrl;
 }