fix: spotify importer uses same iTunes lookup as global search for artwork + metadata

- _loadSpotifyTrackArtwork: rewritten to call itunesSearch() directly from browser (same as global search) instead of backend endpoint — same quality, works for all regions
- enriches track objects with collectionName, releaseDate, trackNumber, trackExplicitness so downloads get full metadata too
- 8-concurrent batch to avoid hammering iTunes
- adds onerror fallback to makeGlobalTrackItem img so broken URLs show 🎵 placeholder

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

src/components.js

@@ -71,7 +71,7 @@ export function makeGlobalTrackItem(t, idx, list) {
   el.className = 'gtrack-item';
   const art = t.artworkUrl60 || t.artworkUrl100 || '';
   el.innerHTML = `
-    <div class="gtrack-art">${art ? `<img src="${art}" loading="lazy">` : '<div style="width:100%;height:100%;display:flex;align-items:center;justify-content:center;font-size:22px">🎵</div>'}</div>
+    <div class="gtrack-art">${art ? `<img src="${art}" loading="lazy" onerror="this.parentNode.innerHTML='<div style=\\'width:100%;height:100%;display:flex;align-items:center;justify-content:center;font-size:22px\\'>🎵</div>'">` : '<div style="width:100%;height:100%;display:flex;align-items:center;justify-content:center;font-size:22px">🎵</div>'}</div>
     <div class="gtrack-info">
       <div class="gtrack-title">${esc(t.trackName||t.collectionName||'—')}${t.trackExplicitness==='explicit'?'<span class="explicit-badge">E</span>':''}</div>
       <div class="gtrack-sub">${esc(t.artistName||'—')} · ${esc(t.collectionName||'')} ${t.releaseDate?'· '+new Date(t.releaseDate).getFullYear():''}</div>