Resume offline preparation on launch and show cache readiness

frontend/sw-update.js

@@ -250,14 +250,15 @@
     // Opt-in: without an askStatus the download path runs exactly as before.
     // (It must never be implicit — a probe that waits on a reply would hang
     // forever if the caller injected a setTimeout that never fires.)
-    let precached = false;
+    let precached = false, offlineStatus;
     if (waiting && askStatus) {
       try {
         const status = await askStatus(waiting);
         precached = !!(status && status.ready);
+        offlineStatus = status;
       } catch { precached = false; }
     }
-    if (onPhase) onPhase(precached ? 'ready' : 'downloading');
+    if (onPhase) onPhase(precached ? 'ready' : 'downloading', offlineStatus);
     if (!precached && refreshShell) await refreshShell();
 
     if (!guard()) throw new Error('Playing - update after this song / when paused');
@@ -280,7 +281,25 @@
     waiting.postMessage({ type: 'SKIP_WAITING' });
   }
 
-  const SwUpdate = { applyUpdate, refreshShellInPlace, askCacheStatus, BUST_PARAM };
+  function showOfflineStatus(status) {
+    const el = root.document?.getElementById('offlineAssetStatus');
+    if (el && typeof status?.total === 'number') {
+      el.textContent = `${status.offlineReady ? 'Offline ready' : 'Preparing offline'} ${status.cached}/${status.total}`;
+    } else if (el && typeof status?.ready === 'boolean') {
+      el.textContent = status.ready ? 'Offline ready (legacy cache)' : 'Offline cache incomplete (legacy mode)';
+    }
+  }
+  async function offlineStatus() {
+    const registration = await root.navigator?.serviceWorker?.getRegistration();
+    const status = await askCacheStatus(registration?.active);
+    showOfflineStatus(status);
+    return status;
+  }
+  root.navigator?.serviceWorker?.addEventListener?.('message', event => {
+    if (event.data?.type === 'OFFLINE_STATUS') showOfflineStatus(event.data);
+  });
+
+  const SwUpdate = { offlineStatus, showOfflineStatus, applyUpdate, refreshShellInPlace, askCacheStatus, BUST_PARAM };
 
   if (typeof module !== 'undefined' && module.exports) {
     module.exports = SwUpdate;