Add padding to icon for visual balance, add screenshots and brand colors to Flathub

[TylerLaBree]

Flathub guidelines suggest that app icons should have padding to help them fit with other app icons.

There's also an issue on the main repo discussing this change.

In this pull request, I simply change the viewBox of the svg from "0 0 1024 1024" to "-96 -96 1216 1216", to follow these guidelines.

[TylerLaBree]

I'm not super familiar with the nuances of Github, so I meant to make these into two pull requests. If you think you'd like to accept one and not the other, I'd be happy to split them up.

The second change is to add screenshots and brand colors to the Flathub listing. Here's a preview of what that would look like.

[cassidyjames]

There's also a discussion I opened at zen-browser/desktop#2646, though that was before the more recent branding refresh I guess.

I would recommend against the matching background color for the dark icon, as that will likely fail the quality check for Flathub (since the icon kind of gets lost there). I'd recommend a slightly lighter color for the dark banner anyway, as the current color could blend too much with the dark background of Flathub and other app store clients.

Here's an example:

<branding>
  <color type="primary" scheme_preference="light">#f2f0e3</color>
  <color type="primary" scheme_preference="dark">#2e2e2e</color>
</branding>