Set up responsive layout for Candid UI auth panel

[rvanasa]

This PR adjusts the style of the new auth UI. I also replaced the "infinity" logo with the monochromatic "astronaut" for consistency with Internet Identity login buttons in other DFINITY dapps.

Context: #475.

Desktop screen width:

Tablet screen width:

Mobile screen width:

[Web3NL]

May I ask, how do you guys locally develop this UI?
(I need to be able see the styling when in the iframe inside Motoko Playground)

@rvanasa only looking at the css, it looks like its (almost) done? Can I improve anything or should I go on to documenting this feature in portal?

[chenyan-dfinity]

Two ways: 1) if you open the UI on a separate window and reduce the width of the browser window, the UI will change to its mini form; 2) To test it in the playground, update this line https://github.com/dfinity/motoko-playground/blob/main/src/config/actor.js#L38 to be the new Candid UI canister id.

[rvanasa]

only looking at the css, it looks like its (almost) done?

I'll take care of the responsive layout. :)

[Web3NL]

only looking at the css, it looks like its (almost) done?

I'll take care of the responsive layout. :)

Great!

I will make sure this is properly documented.