1440 is probably a "safe" minimum. I believe most displays that size would be 1920, these days.
For your app, though, I don't think you can safely assume a certain pixel width. Modern displays come with a dizzying array of resolutions and densities.
Instead, try to design your pages to respond gracefully to any screen width. Using typography-based units like
ch, you can set up your pages to lay themselves out according to the content, so that the design is more flexible. For wide data tables, you will probably need to set a reasonable maximum width to aid readability, in addition to handling how the app responds when the viewport is smaller than you expect. (Even on a 24" monitor, browser windows could be any size at or below their resolution.)