The service worker's notificationclick handler called
self.clients.openWindow(url) unconditionally, spawning a new window/PWA
instance on every notification click even when one was already open.
Focus an existing window client and navigate it to the (relative)
deep-link, resolved against self.location.origin, falling back to
openWindow only when no window is available.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
