Service worker: call notification.close() on notificationclick so OS tray clears after open

Problem/Motivation

On Android Chrome, tapping a Web Push notification opens the target URL, including in PWA standalone mode, but the notification remains visible in the system notification tray/shade. The bug is most consistently reproduced when the notification link targets a PWA, where clients.openWindow() behaviour differs from a regular browser tab.

Steps to reproduce

Prerequisites: drupal/pwa module installed and enabled; site configured as a PWA.

1. Install the site as a PWA on Android Chrome and subscribe to Web Push.
2. Trigger a push that uses service_worker_notification.js (showNotification + notificationclick handler).
3. Tap the notification when it appears.
4. Observe: the PWA opens the URL, but the notification entry remains in the notification tray/shade until dismissed manually.

Proposed resolution

In js/service_worker_notification.js, inside the notificationclick listener, call event.notification.close() before event.waitUntil(...) so the notification is explicitly dismissed when the user acts on it. This matches common Service Worker / Notification API guidance for click handling.

Remaining tasks

5. Maintainer review for any platform-specific concerns.
6. Confirm behaviour on Android Chrome (primary target) and desktop Chrome.

User interface changes

None (behavioural fix only).

API changes

None.

Data model changes

None.