feat(mobile): add FCM push notification when a new freeze proposal is created

[omolobamoyinoluwa-max]

Summary

The mobile app polls for proposals on open. Admins should receive a push notification when a new proposal is created so they can approve without opening the app.

Acceptance criteria

• Backend sends FCM notification on first vote for a new proposal
• Mobile app handles onMessage and navigates to AlertScreen
• Notification includes asset code and target address