Fix stuck state when permission granted but no subscription

When you had already granted notification permission but hadn't
completed the subscription flow (no service worker or no push
subscription), the UI showed neither the subscribe button nor the
enabled state, leaving you stuck with no way to subscribe, and wrong
instructions to fix it.

Instead, let's just show the button to allow you to subscribe.

app/javascript/controllers/notifications_controller.js

@@ -11,8 +11,7 @@ export default class extends Controller {
 
     switch(Notification.permission) {
       case "default":
-        this.subscribeButtonTarget.hidden = false
-        this.explainerTarget.hidden = true
+        this.#showButtonToSubscribe()
         break
       case "granted":
         const registration = await this.#getServiceWorkerRegistration()
@@ -20,6 +19,8 @@ export default class extends Controller {
 
         if (registration && subscription) {
           this.element.classList.add(this.enabledClass)
+        } else {
+          this.#showButtonToSubscribe()
         }
         break
     }
@@ -76,6 +77,11 @@ export default class extends Controller {
     }
   }
 
+  #showButtonToSubscribe() {
+    this.subscribeButtonTarget.hidden = false
+    this.explainerTarget.hidden = true
+  }
+
   async #requestPermissionAndSubscribe(registration) {
     const permission = await Notification.requestPermission()
     if (permission === "granted") this.#subscribe(registration)