The Atbash cipher replaces letters as if the alphabet were mirrored: the first letter becomes the last, the second becomes the second-to-last, and so on. The function encode receives a string plain, converts letters to lowercase, and replaces them using the rule a ↔ z, b ↔ y. Digits stay unchanged, and the final encoded text should be grouped into blocks of 5 characters.

def encode(plain: str) -> str: