feat: album art fallback for artist grid view

Artists without dedicated images (from fanart.tv, TheAudioDB,
Wikidata, etc) now fall back to their most popular album's cover
art in the library artist grid. Uses the appropriate size tier
based on device pixel ratio — CoverArtSmall for small avatars,
CoverArtMedium/Large for larger ones. Only letter-initial
placeholder remains as the absolute last resort.

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

@@ -18,6 +18,7 @@ import {
 } from '@go/library/Library';
 import { library } from '@go/models';
 import { LibraryController } from '@store/controllers/library-controller';
+import { libraryStore } from '@store/library-store';
 import { SearchController } from '@store/controllers/search-controller';
 import { queueStore } from '@store/queue-store';
 import {
@@ -995,6 +996,26 @@ export class ArtistsView
             imageURL = artist.ImageLarge || '';
         }
 
+        // Fallback: use album cover art if no artist image.
+        if (!imageURL) {
+            const cachedAlbums = libraryStore.cachedAlbums;
+            if (cachedAlbums) {
+                const name = artist.Name.toLowerCase();
+
+                for (const a of cachedAlbums) {
+                    if (a.ArtistName.toLowerCase() === name) {
+                        if (needed <= 100) {
+                            imageURL = a.CoverArtSmall || a.CoverArtMedium || '';
+                        } else {
+                            imageURL = a.CoverArtMedium || a.CoverArtLarge || '';
+                        }
+
+                        if (imageURL) break;
+                    }
+                }
+            }
+        }
+
         if (imageURL) {
             return html`<img
                 class="avatar-image"