There are existing projects like CyberChef and DevUtils, but I wanted an offline app that was at least eventually open-source. This also gave me an opportunity to explore Flutter for cross-platform desktop apps. Flutter gives the ability to build native GUI apps for Windows, Linux, MacOS _and_ Web (in case organizations want to self-host it internally for their entire team). The value proposition is unresistable, because the only real alternative seems to be Electron.
Mac and Windows builds have been tested. The UI style is standard from the macos_ui package.
So far, Flutter has been a pleasant experience but the desktop platform seems not quite ready for prime time. For example, webviews are only supported on Android and iOS.