feat: top releases sorted by popularity with library-style album cards

Replaced the date-sorted release group approach with a dedicated
TopReleaseGroupsForArtist LB API call that returns releases ranked
by total listen count (popularity). Added LBTopReleaseGroup type
and Wails bindings.

Restyled the top-releases cards to match the library album view:
square cover art on top with title and type below, centered text,
auto-filling the available width with even spacing.

backend/explore/explore.go

@@ -184,6 +184,11 @@ func (e *Service) TopRecordingsForArtist(artistMBID string) ([]LBTopRecording, e
 	return e.lb.TopRecordingsForArtist(e.ctx, artistMBID)
 }
 
+// TopReleaseGroupsForArtist returns the most-listened release groups for an artist.
+func (e *Service) TopReleaseGroupsForArtist(artistMBID string) ([]LBTopReleaseGroup, error) {
+	return e.lb.TopReleaseGroupsForArtist(e.ctx, artistMBID)
+}
+
 // SimilarArtists returns artists similar to the given artist MBID.
 func (e *Service) SimilarArtists(artistMBID string) ([]LBSimilarArtist, error) {
 	return e.lb.SimilarArtists(e.ctx, artistMBID)