feat(spotify): lazy-load per-track iTunes artwork for playlists/albums

Adds POST /api/spotify/track-artwork endpoint that takes an array of
{idx, trackName, artistName} and returns iTunes artwork URLs in batches
of 8 concurrent requests.

Frontend calls this after rendering the track list, then patches each
.gtrack-art element with the correct album cover as results arrive.
Also updates _spotifyResolved.tracks so downloads use the right artwork.

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

src/views.js

@@ -1043,6 +1043,39 @@ function _renderSpotifyPreview(data) {
   list.innerHTML = '';
   (data.tracks || []).forEach((t, i) => list.appendChild(makeGlobalTrackItem(t, i, data.tracks)));
   section.style.display = '';
+
+  // Lazy-load correct per-track artwork via iTunes in the background
+  if ((data.type === 'playlist' || data.type === 'album') && count > 0 && S.cfg.backendUrl) {
+    _loadSpotifyTrackArtwork(data.tracks, list);
+  }
+}
+
+async function _loadSpotifyTrackArtwork(tracks, listEl) {
+  try {
+    const payload = tracks.map((t, i) => ({ idx: i, trackName: t.trackName, artistName: t.artistName }));
+    const res = await fetch(`${S.cfg.backendUrl}/api/spotify/track-artwork`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ tracks: payload }),
+    });
+    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
+      if (window._spotifyResolved?.tracks?.[r.idx]) {
+        window._spotifyResolved.tracks[r.idx].artworkUrl100 = r.artworkUrl;
+      }
+    }
+  } catch { /* artwork loading is non-critical */ }
 }
 
 // ─── HELPERS ───