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

@@ -1,7 +1,8 @@
 import { LitElement, html, css, nothing } from 'lit';
 import { customElement, state, query as litQuery } from 'lit/decorators.js';
 import { designTokens } from '../../styles/tokens.css';
-import { Search, GetThumbnail } from '@go/explore/Service';
+import { Search, GetThumbnails } from '@go/explore/Service';
+import type { ThumbnailRequest } from '@go/explore/Service';
 import type {
     MBSearchResult,
     MBArtist,
@@ -566,6 +567,8 @@ export class ExploreView extends LitElement {
             }
 
             this.results = result;
+            this.loadThumbnails();
+
             const elapsed = (performance.now() - startTime).toFixed(0);
             console.log(
                 `[explore] search completed: "${query}" in ${elapsed}ms — ` +
@@ -587,21 +590,67 @@ export class ExploreView extends LitElement {
 
     /* ── Thumbnail Loading ── */
 
-    private loadThumbnail(mbid: string, albumName: string, artistName: string) {
-        // Don't re-fetch if already loading or cached.
-        if (this.thumbnailCache.has(mbid)) return;
+    private thumbnailBatchPending = false;
 
-        // Mark as loading to prevent duplicate requests.
-        this.thumbnailCache.set(mbid, '');
+    /**
+     * Load thumbnails for all visible album cards in one batched
+     * Wails call.  Called after search results are set.
+     */
+    private loadThumbnails() {
+        if (this.thumbnailBatchPending || !this.results?.releaseGroups?.length) {
+            return;
+        }
 
-        GetThumbnail(mbid, albumName || '', artistName || '').then((dataUrl) => {
-            if (dataUrl) {
-                this.thumbnailCache.set(mbid, dataUrl);
-                this.requestUpdate();
+        // Collect MBIDs that need fetching.
+        const requests: ThumbnailRequest[] = [];
+
+        for (const rg of this.results.releaseGroups) {
+            if (!this.thumbnailCache.has(rg.mbid)) {
+                requests.push({
+                    mbid: rg.mbid,
+                    albumName: rg.title || '',
+                    artistName: rg.artistCredit || '',
+                });
             }
-        }).catch(() => {
-            // Leave empty string in cache — fallback will show.
-        });
+        }
+
+        if (requests.length === 0) return;
+
+        this.thumbnailBatchPending = true;
+
+        GetThumbnails(requests)
+            .then((results) => {
+                let updated = false;
+
+                for (const [mbid, dataUrl] of Object.entries(results)) {
+                    if (dataUrl) {
+                        this.thumbnailCache.set(mbid, dataUrl);
+                        updated = true;
+                    }
+                }
+
+                // Mark MBIDs with no art so we don't re-request.
+                for (const req of requests) {
+                    if (!this.thumbnailCache.has(req.mbid)) {
+                        this.thumbnailCache.set(req.mbid, '');
+                    }
+                }
+
+                if (updated) {
+                    this.requestUpdate();
+                }
+            })
+            .catch(() => {
+                // Batch failed — mark all as attempted.
+                for (const req of requests) {
+                    if (!this.thumbnailCache.has(req.mbid)) {
+                        this.thumbnailCache.set(req.mbid, '');
+                    }
+                }
+            })
+            .finally(() => {
+                this.thumbnailBatchPending = false;
+            });
     }
 
     /* ── Top Results ── */
@@ -892,11 +941,6 @@ export class ExploreView extends LitElement {
                         const artURL = cachedArt || CoverArtGroupURL(rg.mbid);
                         const year = extractYear(rg.firstReleaseDate);
 
-                        // Kick off async thumbnail fetch if not cached.
-                        if (!cachedArt) {
-                            this.loadThumbnail(rg.mbid, rg.title, rg.artistCredit);
-                        }
-
                         return html`
                             <div
                                 class="album-card"

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);
 }