Improve Installer UX

[TekExplorer]

Blocked by #1

The UX of the installer is mediocre at best.

This is not because the dev is bad at UI, no. it was built in literally 3 days, or so the commits suggest. In fact, I would argue that its instead proof of how awesome flutter is that you can just throw this sort of thing together in 3 days!

Its functional. However, it can be better.

This is primarily a notice to myself to come back to this after the holidays, as I am a Flutter dev myself.

Notes:

• None of the platform folders except for Windows and Linux are needed, unless MacOS is meant to be supported.
• Dark Mode, of course.
• Heckify UI (icons and stuff)
• Code signing?
• Package into single executable?
• Detect installed game?