Merge fildev-payment-client (demo feature branch) to main

[geoknee]

It's this PR #1672

• Update netlify config to deploy the demo site from main branch (and skip if the relevant package was not changed).
• perform a final review of the entire changeset (bearing in mind it has already been reviewed already, although potentially under greater time pressure than usual).
• consider whether we need multiple bootpeers specified (I think it should be just iris)
• consider whether we want to keep any of the code in a separate branch and not merge to main (The advantages don't seem huge to me, and I think we should just merge it all, and then we can reconfigure things later if we want)
• merge outstanding PRs into main when their base gets automatically changed
• ensure demo still works