- Add VITE_EMAIL_NOTIFICATION_SERVER env var
- Extend notificationSettings with email, emailFrequency, emailAddress
- Add emailNotificationState store in push/adapters/common.ts
- New src/app/mail/index.ts with MailNotifications class:
register/unregister with mailship via NIP-98 auth,
publish kind 30390 NIP-9a subscription events
- Wired into alerts settings save handler
- Email Digest Settings card in alerts UI
