1. Device queue
Browser stores events locally in IndexedDB when offline.
2. Sync push
When online, the browser posts events to the server API.
3. Idempotency
Each event has UUID/idempotency key to avoid duplicates.
4. Conflict rules
Server will apply safe events and flag conflicts for manager review.
This phase creates the safe foundation. Future phases will decide which POS actions are allowed offline. Payments should remain online or processor-tokenized unless a compliant offline-payment strategy is chosen.