There is the completely open source https://standardnotes.com/ which would support that via their syncing and the authenticator note type.
Unfortunately it does not look like their free plan allows you to use that note type. So could also host it yourself though (and pay for the premium token their or hack it out - it's foss). Have never done that myself though
Well yeah but imagine you had to do that on most lines of the code? It would become very distracting imho. If you are in a team with people that have a lot experience and or will learn more anyway this is fine. But if you are in a team with not very good programmers which "will never learn" because they have other stuff to do, you should be careful when using code like this. Though I would prefer in the former of course.
... can't you just put i straight back into the sea?
Replace "server" by "os", "computer" or "software project" and you got me as well.
Why exactly is that? Because it's reduced security?