I have developed a system-wide writing assistant like you're describing. By design, it has no exceptions to where it works.

Currently, it's only for Mac, but I'm working on an Electron version too (though it's quite challenging).

Check out https://steerapp.ai/

Is the Electron version supposed to be available on Linux? I see only mentions of Windows on the website.

Would love to see the code, any plans on open sourcing?

yes! its open sourced right now: https://github.com/MuffinTheDragon/daily-habit-tracker

I was wondering whether the table actually comes from some paper, or it's just a marketing trick for techy folks.