perf: batch thumbnail loading — one Wails call for all album art

Replace per-card GetThumbnail calls (10 round-trips) with a single
GetThumbnails batch call that fetches all visible album thumbnails
in one Wails bridge round-trip.

Backend GetThumbnails accepts []ThumbnailRequest and returns
map[mbid]→dataURL. Each request still checks library → disk cache
→ CAA in order, but the bridge overhead is 1 call instead of 10.

Frontend fires loadThumbnails() once after search results arrive.
Album cards render immediately with CAA URL fallback, then re-render
once the batch resolves with cached/local data URLs.

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

@@ -13,6 +13,14 @@ export function CoverArtURL(arg1:string):Promise<string>;
 
 export function GetThumbnail(arg1:string, arg2:string, arg3:string):Promise<string>;
 
+export interface ThumbnailRequest {
+  mbid: string;
+  albumName: string;
+  artistName: string;
+}
+
+export function GetThumbnails(arg1:ThumbnailRequest[]):Promise<Record<string, string>>;
+
 export function LookupArtist(arg1:string):Promise<explore.MBArtist>;
 
 export function LookupReleaseGroup(arg1:string):Promise<explore.MBReleaseGroup>;

frontend/wailsjs/go/explore/Service.js

@@ -22,6 +22,10 @@ export function GetThumbnail(arg1, arg2, arg3) {
   return window['go']['explore']['Service']['GetThumbnail'](arg1, arg2, arg3);
 }
 
+export function GetThumbnails(arg1) {
+  return window['go']['explore']['Service']['GetThumbnails'](arg1);
+}
+
 export function LookupArtist(arg1) {
   return window['go']['explore']['Service']['LookupArtist'](arg1);
 }