feat: migrate gui to uv tool

[HarlanHeilman]

To prevent the need for complex import logic, the GUI could be shipped as a tool that can be installed using uv tool install kkcalc.

Here is some documentation on this https://docs.astral.sh/uv/concepts/tools/#tool-versions

[mattgebert]

Thanks @HarlanHeilman, hoping to push this package towards a proper release shortly. Just back from parental leave.

I've actually been able to use pyinstaller to automatically geneate GUI executables, though specifying the scripts in the build tools (see hatch/hatchling gui-script installers here) should provide easy install once the new package is on PyPI.

If this is easy to setup, happy to implement as well.