Investigate cross-device notification dismissal alternatives

[emilhauk]

Context

We removed the cross-device push notification dismissal mechanism (clear pushes) in the commit that references this issue. When a user opened a room on one device, the server sent data-only "clear" Web Push notifications to all other subscriptions to dismiss stale notifications. This has been removed.

Why it was removed

1. Chrome push budget penalty — Chrome penalizes silent pushes (pushes that don't result in user-visible notifications). Our suppressNotification() workaround (show a notification and immediately close it) is fragile and still counts against the budget.
2. suppressNotification() fragility — The 100ms show-then-close timing is a race condition. If Chrome changes its internal behavior, this breaks silently.
3. Safari differences — Safari doesn't have the same silent push restrictions, but the workaround is Chrome-specific and adds complexity for all platforms.

What still works

• Same-device clearing via postMessage from the active tab to the Service Worker. This is free, instant, and reliable.

Alternative approaches to investigate

• TTL-based expiry — Set short TTL on push notifications so they auto-expire. Doesn't solve cross-device clearing but reduces stale notification window.
• Tag-based replacement — Use consistent tags so new notifications replace old ones on the same device. Already partially in place.
• Background Sync API — Register a sync event when the user opens a room; the SW can then clear notifications without a push. Limited browser support.
• Periodic Background Sync — Periodically check server for read state and clear notifications. Very limited browser support (Chrome only, requires installed PWA).
• Accept the trade-off — Notifications on other devices persist until tapped or manually dismissed. This is the current behavior of most chat apps on web.