Keep push notifications on screen until dismissed

Without requireInteraction, the OS default auto-dismiss (a few seconds
on most platforms) was closing notifications before they were
reliably noticed. Browser/PWA push only -- the Desktop bridge is a
separate codebase with its own native notification handling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/sw.ts

@@ -115,6 +115,10 @@ self.addEventListener('push', (event) => {
       icon: '/icons/icon-192.png',
       badge: '/icons/icon-192.png',
       data: { room_id: payload.room_id },
+      // Without this, the OS default auto-dismiss (a few seconds on most
+      // platforms) closes it long before anyone reliably notices -- stays
+      // on screen until the user actually dismisses or clicks it instead.
+      requireInteraction: true,
     }),
   )
 })