Schema-driven configuration Smart Cell for AtpClient backends.

Iterates AtpClient.backends/0, renders each backend's AtpClient.Backend.config_schema/0 as a grouped form (Connection / Defaults / Advanced), and emits one Application.put_env/3 call per backend the user has filled in. Includes a Verify button that calls AtpClient.Backend.verify/1 so the cell can flag bad credentials before the rest of the notebook tries to use them.