[Feature] - Responsive main screen

[YonatanKra]

When opening the screen on several laptops, it looks and feels strange.
I get two levels of scrolling (main page and the new/join game section). Both are cut by like 100-200 pixels (depends on screen size) which is annoying - especially when most of the screen isn't used at all.
Using flex correctly usually avoids such issues "out of the box".
In addition, it might also improve performance in #214 (didn't check deep enough, so it's just a speculation).