Add `pyproject.toml` for the Linux desktop python package

[tastelessjolt]

Description

I added the pyproject.toml file so that the package + dependencies can be install as a tool using uv tool install ClipCascade_Desktop/src after cloning the repo.

You might need to run uv tool update-shell to make sure the installed tools are added to your PATH.

Once installed as a tool, you can just run clipcascade to start the application.

Currently, the pyproject.toml uses the requirements_linux.txt hardcoded. Ideally, it should select the proper requirements file depending on the platform. I'll update this part once I test it on other platforms.