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)

backend/explore/listenbrainz.go

@@ -97,6 +97,52 @@ func (c *ListenBrainzClient) TopRecordingsForArtist(
 	return out, nil
 }
 
+// TopReleaseGroupsForArtist returns the most-listened release groups
+// for the artist identified by artistMBID.
+func (c *ListenBrainzClient) TopReleaseGroupsForArtist(
+	ctx context.Context, artistMBID string,
+) ([]LBTopReleaseGroup, error) {
+	url := fmt.Sprintf(
+		"%s/1/popularity/top-release-groups-for-artist/%s",
+		listenBrainzBaseURL,
+		artistMBID,
+	)
+	cacheKey := "lb:top-release-groups:" + artistMBID
+
+	if data, ok := c.cache.Get(cacheKey); ok {
+		var out []LBTopReleaseGroup
+		if err := json.Unmarshal(data, &out); err == nil {
+			return out, nil
+		}
+	}
+
+	body, err := c.doGet(ctx, url)
+	if err != nil {
+		return nil, fmt.Errorf("listenbrainz top release groups: %w", err)
+	}
+
+	var wire []lbTopReleaseGroupWire
+	if err := json.Unmarshal(body, &wire); err != nil {
+		return nil, fmt.Errorf("listenbrainz top release groups unmarshal: %w", err)
+	}
+
+	const maxTopReleaseGroups = 10
+
+	limit := len(wire)
+	if limit > maxTopReleaseGroups {
+		limit = maxTopReleaseGroups
+	}
+
+	out := make([]LBTopReleaseGroup, limit)
+	for i := range limit {
+		out[i] = wire[i].toPublic()
+	}
+
+	c.cacheJSON(cacheKey, out, cacheTTLSearch, artistMBID, "artist")
+
+	return out, nil
+}
+
 // SimilarArtists returns artists similar to the one identified by
 // artistMBID, using the ListenBrainz labs API.  Returns nil, nil
 // if the endpoint is unavailable (labs API may be unstable).

backend/explore/types.go

@@ -106,3 +106,45 @@ type LBSimilarArtist struct {
 	Name       string  `json:"name"`
 	Score      float64 `json:"score"`
 }
+
+// LBTopReleaseGroup represents a popular release group from the
+// ListenBrainz popularity API.
+type LBTopReleaseGroup struct {
+	ReleaseGroupMBID string `json:"releaseGroupMbid"`
+	Title            string `json:"title"`
+	ArtistName       string `json:"artistName"`
+	Type             string `json:"type"`
+	TotalListenCount int    `json:"totalListenCount"`
+}
+
+// lbTopReleaseGroupWire matches the ListenBrainz API's snake_case
+// JSON response for the popularity/top-release-groups-for-artist
+// endpoint.
+type lbTopReleaseGroupWire struct {
+	ReleaseGroupMBID string `json:"release_group_mbid"`
+	TotalListenCount int    `json:"total_listen_count"`
+	ReleaseGroup     struct {
+		Name string `json:"name"`
+		Type string `json:"type"`
+	} `json:"release_group"`
+	Artist struct {
+		Artists []struct {
+			Name string `json:"name"`
+		} `json:"artists"`
+	} `json:"artist"`
+}
+
+func (w lbTopReleaseGroupWire) toPublic() LBTopReleaseGroup {
+	artistName := ""
+	if len(w.Artist.Artists) > 0 {
+		artistName = w.Artist.Artists[0].Name
+	}
+
+	return LBTopReleaseGroup{
+		ReleaseGroupMBID: w.ReleaseGroupMBID,
+		Title:            w.ReleaseGroup.Name,
+		ArtistName:       artistName,
+		Type:             w.ReleaseGroup.Type,
+		TotalListenCount: w.TotalListenCount,
+	}
+}