Simulator volume slider

[hchargois]

The buzzer in the simulator plays a triangle wave at full gain, which is extremely loud and annoying, at least to my ears.

This PR adds a volume slider to control the buzzer's volume. The value is saved to the browser's local storage, so it stays the same between reloads of the page.

While I was at it, I added the selected skin in local storage too.

[hchargois]

Ah, I didn't see this before as it only shows up on tiny screens.

I took that opportunity to improve the page design a bit.

Here's a before/after comparison on a phone screen.

[WesleyAC]

Looks great, thanks!