Paint saved playlists before the launch sync and swap a background-downloaded update instantly

frontend/sw.js

@@ -176,6 +176,24 @@ self.addEventListener('message', (e) => {
   if (e.data && e.data.type === 'SKIP_WAITING') {
     self.skipWaiting();
   }
+  // "Is your shell already downloaded?" — a waiting worker has precached the
+  // whole new build during install, so "Refresh UI" can swap to it instantly
+  // instead of downloading everything a second time. Answer honestly: install
+  // deletes its cache on failure, but a cache can also be evicted under
+  // storage pressure, so the files are actually checked.
+  if (e.data && e.data.type === 'CACHE_STATUS') {
+    e.waitUntil((async () => {
+      let missing = SHELL.length;
+      try {
+        const cache = await caches.open(CACHE);
+        const found = await Promise.all(SHELL.map((url) => cache.match(url)));
+        missing = found.filter((r) => !r).length;
+      } catch { /* storage blocked — report not ready */ }
+      const reply = { type: 'CACHE_STATUS', version: VERSION, ready: missing === 0, missing };
+      if (e.ports && e.ports[0]) e.ports[0].postMessage(reply);
+      else if (e.source) e.source.postMessage(reply);
+    })());
+  }
 });
 
 // ============================================================================