fix: handle iTunes rate-limit empty responses, reduce Spotify artwork concurrency

- itunesSearch: use r.text() + JSON.parse so empty body (rate-limit) returns [] instead of throwing
- reduce Spotify artwork batch size 8→4 to avoid hammering iTunes and affecting global search

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

src/api.js

@@ -43,9 +43,12 @@ export async function ndAuth() {
 const ITUNES = 'https://itunes.apple.com';
 
 export async function itunesSearch(query, entity='musicTrack', limit=25) {
+  try {
     const r = await fetch(`${ITUNES}/search?term=${encodeURIComponent(query)}&media=music&entity=${entity}&limit=${limit}&country=us&explicit=Yes`);
-  const d = await r.json();
-  return d.results || [];
+    const text = await r.text();
+    if (!text) return [];
+    return JSON.parse(text).results || [];
+  } catch { return []; }
 }
 
 export async function itunesLookupAlbum(collectionId) {

src/spotify.js

@@ -172,7 +172,7 @@ async function _loadSpotifyTrackArtwork(tracks, listEl) {
   });
 
   // Enrich each track with iTunes data — same call as global search
-  const CONC = 8;
+  const CONC = 4;
   for (let i = 0; i < tracks.length; i += CONC) {
     const batch = tracks.slice(i, i + CONC);
     await Promise.all(batch.map(async (t, bi) => {