Why not make a locally-hosted web page out of it: a local file that you open in your browser, and does the math/magik right there?
(For a graphical example, see e.g.
my finite impulse filter response analysis example. When loaded, the page does not connect to the internet in any way, and works just fine when saved as a local file and simply opened in your browser.)
Personally, I'd consider three inputs – server/domain name, common passphrase, mixing method – and one output, the password generated, when you click on a button.
The mixing methods needs to be cryptographically secure (not secret, secure); and the "randomness" is provided by the passphrase (common across the sites you use this with, or some other phrase you associate with that particular site) that you do need to keep secret.
For the page, use HTML and CSS for the layout, and Javascript (included in the HTML page itself, like in my example above) for the functionality.