Automate fake screenshots

Crazy idea inspired from the olden days of our wallpaper experiment:

1. A bot takes a submitted image from the PR
2. Determines if the top is light or dark
3. Overlays the right version of a transparent desktop PNG on top with the panel and dock
4. Comments on the PR with the image

We could get like 16:9, 16:10, 3:2, and ultrawide versions automatically to see it in a variety of setups.