Make it bigger on desktop

[dfabulich]

https://curiousdannii.github.io/infocom-frotz/journey.html looks very small on my laptop screen.

I think it would be good to let the canvas grow to fit the width of the window

[dfabulich]

This will likely look better if/when we do #4

[curiousdannii]

This is a bit trickier because Frotz itself sets its own size. I had to do some manual hacking to avoid it being wider than the screen on mobile. But yeah, probably no point even attempting it until there's better pixel density support.