fix: spotify importer artwork — split multi-artist before iTunes, render Spotify art immediately

- track-artwork endpoint: split artist on ,/•/ before iTunes search (same as download flow)
- track-artwork endpoint: fall back to existingArt when iTunes returns nothing
- frontend: render any artwork already in Spotify data immediately, don't wait for iTunes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/spotify.js

@@ -160,7 +160,25 @@ function _renderSpotifyPreview(data) {
 
 async function _loadSpotifyTrackArtwork(tracks, listEl) {
   try {
-    const payload = tracks.map((t, i) => ({ idx: i, trackName: t.trackName, artistName: t.artistName }));
+    const artEls = listEl.querySelectorAll('.gtrack-art');
+
+    // Render any artwork already present in the Spotify data immediately
+    tracks.forEach((t, i) => {
+      const url = t.artworkUrl100 || t.artworkUrl60 || '';
+      if (!url) return;
+      const el = artEls[i];
+      if (!el) return;
+      const img = document.createElement('img');
+      img.src = url; img.loading = 'lazy';
+      el.innerHTML = ''; el.appendChild(img);
+    });
+
+    const payload = tracks.map((t, i) => ({
+      idx: i,
+      trackName: t.trackName,
+      artistName: t.artistName,
+      existingArt: t.artworkUrl100 || t.artworkUrl60 || '',
+    }));
     const res = await fetch(`${S.cfg.backendUrl}/api/spotify/track-artwork`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
@@ -168,17 +186,13 @@ async function _loadSpotifyTrackArtwork(tracks, listEl) {
     });
     if (!res.ok) return;
     const { results } = await res.json();
-    const artEls = listEl.querySelectorAll('.gtrack-art');
     for (const r of results) {
       if (!r.artworkUrl || r.idx == null) continue;
       const el = artEls[r.idx];
       if (!el) continue;
       const img = document.createElement('img');
-      img.src = r.artworkUrl;
-      img.loading = 'lazy';
-      el.innerHTML = '';
-      el.appendChild(img);
-      // Update resolved data so downloads use the correct artwork
+      img.src = r.artworkUrl; img.loading = 'lazy';
+      el.innerHTML = ''; el.appendChild(img);
       if (window._spotifyResolved?.tracks?.[r.idx]) {
         window._spotifyResolved.tracks[r.idx].artworkUrl100 = r.artworkUrl;
       }