feat: show total LB play count on artist detail page

Added GetArtistPlayCount(mbid) — fetches ArtistPopularity from LB
for a single MBID and returns the total listen count. Fire-and-forget
call on the artist page, displays below the meta line as
'1.3M plays on ListenBrainz' (uses existing formatListenCount).

frontend/wailsjs/go/explore/Service.js

@@ -34,6 +34,10 @@ export function GetArtistMBID(arg1) {
   return window['go']['explore']['Service']['GetArtistMBID'](arg1);
 }
 
+export function GetArtistPlayCount(arg1) {
+  return window['go']['explore']['Service']['GetArtistPlayCount'](arg1);
+}
+
 export function GetThumbnail(arg1, arg2, arg3) {
   return window['go']['explore']['Service']['GetThumbnail'](arg1, arg2, arg3);
 }