Filter by Substring (Case-Insensitive)

Context. Filter a list of emails to those from a particular domain.

Your task. Write emails_from_domain(df, domain) that returns rows where df["email"] contains domain as a substring (case-insensitive).

Example. domain "gmail" matches "alice@gmail.com" and "carol@GMAIL.com" both.

Notes. Case-insensitive match. No matches → empty DataFrame.