Fix push notification re-subscription when subscriptions expire

[waynemsmith]

Summary

This fixes two issues that prevent users from receiving push notifications after their browser subscription expires:

• Add pushsubscriptionchange handler to service worker - When browsers invalidate/expire a push subscription (Safari does this aggressively, often within 24-72 hours), they fire this event. The handler automatically re-subscribes and syncs with the server.

• Show subscribe button when permission granted but no subscription exists - Previously, if permission was "granted" but the subscription expired, the UI showed "Push notifications are OFF" but no button to re-subscribe. Users were stuck unable to re-enable notifications.

Problem

Safari is known to expire push subscriptions after periods of inactivity. When this happens:

1. The pushsubscriptionchange event fires in the service worker
2. Without a handler, nothing happens client-side
3. Server attempts to push to the old endpoint → Apple rejects it
4. Server catches ExpiredSubscription and deletes the subscription
5. User sees "OFF" but can't re-subscribe because the button is hidden (permission is still "granted")

Test plan

• Enable push notifications in Safari
• Wait for subscription to expire (or manually unsubscribe via DevTools)
• Verify the "Turn ON push notifications" button appears
• Click to re-subscribe
• Verify notifications work again

🤖 Generated with Claude Code

[rosa]

Oops! I hadn't seen this PR when I opened #2421 today. That addressed the 2nd issue: permission is granted, but there's no subscription (or no service worker registered).