fix: login screens jarring

[drewdelano]

Closes #1440

Switches the "callback" route to product-app to fix the background

Uses flexbox to position the footer for product-app

[JeffLowe]

Visual inspection of the login flow on the preview link:
Look much better. Login screen footer anchored at bottom so no whitespace under footer.
The callback screen now uses the site's darker background and styled loader animation.
@drewdelano Question: Is there a way to change the white magic auth screen to darker color? Is that something we have to do in magicauth settings (thus only someone with access can accomplish)?

[drewdelano]

@JeffLowe I suspect so, but I can take a look too.

[JeffLowe]

@drewdelano dont let the magiclink screen block getting this pr's improvements out. We can always circle back (in new ticket) to address the magiclink screen.

[drewdelano]

I talked with @JeffLowe about this on Slack. I'm going to go ahead and merge this. I'll fix the magic link bit in another ticket he's creating now.