offline form behaviour

Site now works offline, still sends analytics, but what of contact form?

* Detect online / offline and switch content depending?
* Offer to send SMS/Email using device links when offline?
* Should this contain a pre-formatted body?
* Is this really a sign of potential mis-match (largely provide internet connected tech)
* Use service-worker to catch POST requests and send at a later time if offline
* How to simulate service-worker state without manual testing

The issue names no files, tests, or entry points. Start by identifying the contact form and existing offline or service-worker implementation, then resolve which offline behavior is wanted and how it should be tested. Done means one agreed behavior is implemented and its offline state can be simulated without manual-only testing.

Written by the indexing model from the issue text.