Improve the notification setting description

The current text has invited a lot of confusion around how notifications
work in NeoChat, because it mentions "push notifications". Some users
take it to mean that somehow the notifications appear in the background,
but that's only supported if built with KUnifiedPush.

To make it super clear, let's change the description dynamically based
on whether:
1. NeoChat is built with KUnifiedPush support.
2. We were able to connect with the KUnifiedPush daemon and your server
has a push gateway.

src/neochatconnection.cpp

@@ -472,9 +472,12 @@ QCoro::Task<void> NeoChatConnection::setupPushNotifications(QString endpoint)
                                false);
 
         qInfo() << "Registered for push notifications";
+        m_pushNotificationsEnabled = true;
     } else {
         qWarning() << "There's no gateway, not setting up push notifications.";
+        m_pushNotificationsEnabled = false;
     }
+    Q_EMIT enablePushNotificationsChanged();
 #else
     Q_UNUSED(endpoint)
     co_return;
@@ -536,4 +539,18 @@ bool NeoChatConnection::canEraseData() const
     return m_canEraseData;
 }
 
+bool NeoChatConnection::pushNotificationsAvailable() const
+{
+#ifdef HAVE_KUNIFIEDPUSH
+    return true;
+#else
+    return false;
+#endif
+}
+
+bool NeoChatConnection::enablePushNotifications() const
+{
+    return m_pushNotificationsEnabled;
+}
+
 #include "moc_neochatconnection.cpp"