feat: artist images from MusicBrainz/Wikidata/Wikimedia Commons

Add ArtistImageProvider that resolves artist MBIDs to photo URLs:
1. MB url-rels 'image' type → extract Commons filename → thumb URL
2. MB url-rels 'wikidata' type → Wikidata P18 property → thumb URL
3. No image → falls back to initial-letter avatar

Wikimedia Commons thumb URLs constructed via MD5 hash bucketing
(standard Commons URL scheme). Results cached in explore_cache
with 30-day TTL — subsequent lookups are instant.

Frontend: search results and artist detail page show artist photos
in the circular avatar when available. Images load async and
replace the initial-letter fallback on arrival. Artist detail page
fires the image fetch alongside the other 4 parallel data loads.

Architecture supports adding more sources (fanart.tv, etc.) by
extending the resolve() method's source chain.

frontend/wailsjs/go/explore/Service.js

@@ -18,6 +18,10 @@ export function CoverArtURL(arg1) {
   return window['go']['explore']['Service']['CoverArtURL'](arg1);
 }
 
+export function GetArtistImageURL(arg1) {
+  return window['go']['explore']['Service']['GetArtistImageURL'](arg1);
+}
+
 export function GetThumbnail(arg1, arg2, arg3) {
   return window['go']['explore']['Service']['GetThumbnail'](arg1, arg2, arg3);
 }