Offline-first sync for mobile

Description

Design, implement, and review offline-first data and synchronization for React Native/Expo apps — choosing local storage (expo-sqlite, op-sqlite, WatermelonDB, PowerSync with Supabase, Legend State, TanStack Query persistence), mutation outbox with client-generated UUIDs, idempotent server endpoints, pull sync with cursors and tombstones for deletes, conflict resolution (server-authoritative, last-write-wins with server timestamps, field-level merge, CRDTs), clock skew, on-device schema migrations, encryption at rest, connectivity detection, background sync limits, and testing offline scenarios. Use whenever the user wants an app to work offline, sync local data with a backend, handle conflicts, or reviews sync code.