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/types.ts

@@ -44,6 +44,8 @@ export interface Album {
   artistId: string;
   artistName: string;
   artUrl?: string;
+  /** Whether the album has cover art served by `GET /albums/{id}/cover`. */
+  hasCover: boolean;
   year?: number;
   trackCount: number;
   totalDurationMs: number;