Resume offline preparation on launch and show cache readiness

frontend/app.js

@@ -8690,11 +8690,11 @@ async function applyUpdate(target, { quiet = false } = {}) {
       // When the waiting worker already downloaded this build in the
       // background, the toast says so and nothing is re-downloaded.
       askStatus: (worker) => SU.askCacheStatus(worker, { timeoutMs: 4000, setTimeout: (fn, ms) => setTimeout(fn, ms) }),
-      onPhase: (phase) => {
+      onPhase: (phase, status) => {
         if (quiet) return;
         if (_updateToast) _updateToast.remove();
         _updateToast = phase === 'ready'
-          ? toast('Applying the update…', { duration: 20000 })
+          ? toast(status?.offlineReady === false ? `Applying the update… Offline cache is still preparing (${status.cached}/${status.total}).` : 'Applying the update…', { duration: 20000 })
           : toast('Downloading the update…', { duration: 305000 });
       },
       refreshShell: ('caches' in window) ? () => SU.refreshShellInPlace({