Parse email header

Write a function `parse_email_header(header: str) -> dict` that takes a raw email header as a string and returns a dictionary with keys `'from'`, `'to'`, and `'subject'`. The header uses standard email header format: each line is `Field: value` with the field name followed by a colon and a space. The fields are case-insensitive (e.g., 'From:', 'FROM:', 'from:' are all valid). The string may contain extra whitespace around the value and may have trailing newlines. You may assume every header contains exactly one 'From:', one 'To:', and one 'Subject:' field. Lines that are not one of these three fields should be ignored. Return the values trimmed of leading and trailing whitespace. The dictionary keys must be exactly 'from', 'to', 'subject' (lowercase).

>>> parse_email_header('From: alice@example.com\nTo: bob@example.com\nSubject: Hello')
{'from': 'alice@example.com', 'to': 'bob@example.com', 'subject': 'Hello'}
>>> parse_email_header('SUBJECT: Meeting\nFROM: jane@x.io\nTO: team@x.io\nDate: today')
{'from': 'jane@x.io', 'to': 'team@x.io', 'subject': 'Meeting'}