Add support for opening .designspace files through File Explorer

Hello,

We can drag and drop a .designspace file in FontraPak but it would also be nice to be able to open them by double-clicking on them in the File Explorer on Windows. I can already associate the FontraPak .exe to open .designspace files but when I double-click on a file in the File Explorer it just opens Fontra Pak without doing anything. It should also open the browser to show me the file.

https://github.com/user-attachments/assets/a9c8c5aa-a1cc-4b3f-b7a6-b86d16a383bb

Thank you in advance.