How do you design an offline-first mobile app with reliable sync?

Offline-first apps treat the local database as the source of truth and sync changes in the background. Key pieces: - Store data locally (SQLite/Room/Core Data/Realm) - Track local mutations (outbox queue) - Use conflict resolution (last-write-wins, merge rules, or server authority) - Support retries with backoff and idempotent requests Good UX: clear offline indicators, graceful error states, and deterministic sync behavior.