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.

frontend/wailsjs/go/explore/Service.d.ts

@@ -50,3 +50,5 @@ export function StartIndexBuild():Promise<void>;
 export function StopIndexBuild():Promise<void>;
 
 export function TopRecordingsForArtist(arg1:string):Promise<Array<explore.LBTopRecording>>;
+
+export function TopReleaseGroupsForArtist(arg1:string):Promise<Array<explore.LBTopReleaseGroup>>;

frontend/wailsjs/go/explore/Service.js

@@ -97,3 +97,7 @@ export function StopIndexBuild() {
 export function TopRecordingsForArtist(arg1) {
   return window['go']['explore']['Service']['TopRecordingsForArtist'](arg1);
 }
+
+export function TopReleaseGroupsForArtist(arg1) {
+  return window['go']['explore']['Service']['TopReleaseGroupsForArtist'](arg1);
+}