feat: artist image disk cache + fix top results artist photos

Two changes:

1. Artist image disk cache: ArtistImageProvider now fetches the
   actual image bytes from Wikimedia Commons and caches them on
   disk (~/.local/share/yellowjacket/artist-image-cache/{mbid}.jpg).
   Returns base64 data URLs, same pattern as CoverArtProxy.
   First lookup: resolve URL via MB/Wikidata + fetch image (~2s).
   Subsequent: instant from disk cache.
   404s cached as empty files to avoid re-fetching.

2. Top results artist photos: the Top Results section now shows
   artist images from the artistImageCache, same as the Artists
   section. Also shows englishName in the top card display name.

frontend/src/components/explore-view/explore-view.ts

@@ -866,6 +866,7 @@ export class ExploreView extends LitElement {
         if (item.type === 'artist' && item.artist) {
             const a = item.artist;
             const hue = nameToHue(a.name);
+            const imgURL = this.artistImageCache.get(a.mbid);
             return html`
                 <div
                     class="top-card"
@@ -883,10 +884,12 @@ export class ExploreView extends LitElement {
                         class="artist-avatar"
                         style="background: hsl(${hue}, 45%, 35%)"
                     >
-                        ${(a.englishName || a.name).charAt(0).toUpperCase()}
+                        ${imgURL
+                            ? html`<img src="${imgURL}" alt="${a.englishName || a.name}" />`
+                            : (a.englishName || a.name).charAt(0).toUpperCase()}
                     </div>
                     <div class="top-card-info">
-                        <div class="top-card-name">${a.name}</div>
+                        <div class="top-card-name">${a.englishName || a.name}</div>
                         <div class="top-card-meta">Artist${a.country ? ` · ${a.country}` : ''}</div>
                     </div>
                 </div>