Add an install method for Linux other than Flatpak

`flatpak install` connects to flathub even when installing a local file; as flathub likes to go down it makes it impossible to install the game sometimes

<img width="848" height="507" alt="Image" src="https://github.com/user-attachments/assets/8bbe9972-3886-4df6-9b81-6faa4cf86c05" />