fix(library): show album cover art in the Albums grid

The album cards always fell back to the 💿 placeholder: the mapper dropped
the backend's `has_cover` and no album cover URL was ever built. Carry
`hasCover` through `RawAlbum`/`Album` and add `getAlbumCoverUrl` (GET
/albums/{id}/cover, token in the query like the track/stream URLs). The
Library Albums grid and the artist-detail discography now render real
covers, same source the album-detail page already used.

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

src/api/endpoints/streaming.ts

@@ -32,3 +32,18 @@ export function getTrackCoverUrl(
   const base = getApiBaseUrl();
   return `${base}/tracks/${trackId}/cover?token=${encodeURIComponent(token)}`;
 }
+
+/**
+ * Cover image URL for an album, served by `GET /albums/{id}/cover`. Same
+ * `?token=` rationale as the track cover. Returns undefined when the album has
+ * no cover (so callers fall back to generated tile art).
+ */
+export function getAlbumCoverUrl(
+  albumId: string,
+  token: string,
+  hasCover: boolean,
+): string | undefined {
+  if (!hasCover) return undefined;
+  const base = getApiBaseUrl();
+  return `${base}/albums/${albumId}/cover?token=${encodeURIComponent(token)}`;
+}