Fix image alignment in ready page in some contexts

[Madeorsk]

Problem

Images in buttons are not centered. I think it only occurs when setting up React Admin without using create-react-admin. I saw this using shadcn-admin-kit.

Solution

Add margin auto and ensure display block (display block wasn't actually required in my case but it's probably safer to enforce it).

How To Test

Setup a shadcn admin kit project with this app:

import { Admin } from "@/components/admin";

function App() {
  return <Admin></Admin>;
}

export default App;

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• [ ] The PR includes unit tests (if not possible, describe why)
• [ ] The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date