Letters were completely absent from the notification pipeline — users had
to visit the Letters page to discover incoming letters. This integrates
kind 8211 into every layer of the notification system:
- useNotifications: query, grouping, and referenced-event exclusion
- useHasUnreadNotifications: unread dot indicator
- NotificationsPage: LetterNotification component with link to /letters
- NotificationSettings: toggleable Letters row
- notificationTemplates: web push template
- Android NotificationRelayService + NostrPoller: native push support
- EncryptedSettings + schema: letters preference field
Closes#188
Mary Kate Fain
f82adab05d