Gitlab CI with some remote runners on a VPS that I already have (so I can keep myself on the free tier). Works perfectly.
In a video game the "server" doesn't figure out what's changed (that's too much work) it just redraws the frame. So rather than the complexity of working out what has changed you can just send down the whole page again whenever something changes.
This gives you view = f(state) over the wire and has great DX. It's called immediate mode in games.
Fastmail do have web and mobile clients. On web and iOS, both are a "webapp" disguised as an app (my guess Android version is the same).