Wait for service worker to be active before subscribing

The push subscription requires an active service worker. When
registering a new service worker, we now wait for it to become
active using navigator.serviceWorker.ready before attempting
to subscribe to push notifications.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/javascript/controllers/notifications_controller.js

@@ -55,8 +55,9 @@ export default class extends Controller {
     return navigator.serviceWorker.getRegistration("/service-worker.js", { scope: "/" })
   }
 
-  #registerServiceWorker() {
-    return navigator.serviceWorker.register("/service-worker.js", { scope: "/" })
+  async #registerServiceWorker() {
+    await navigator.serviceWorker.register("/service-worker.js", { scope: "/" })
+    return navigator.serviceWorker.ready
   }
 
   async #subscribe(registration) {