Fix the sign-up field check for global users

An AI added validation to your global product's sign-up form. Its rules rejected names with anything but letters, demanded a US phone format and used a strict email pattern. The tests passed, because they used the team's own names. Your team is replacing it with a validator that checks only what it must, so nobody is turned away for having a real name or a foreign number.

An assistant wrote check_registration_field(field_kind, value). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• field_kind: "name", "email" or "phone". Any other value is unknown.
• value: what the person typed.
• Return one status: "ok", "empty", "invalid_email", "too_few_digits" or "unknown_field".

Rules the tests check, in order:

1. An unknown field_kind returns "unknown_field", whatever the value.
2. Trim spaces and tabs, and only those two characters, from both ends of the value. If nothing is left, return "empty".
3. A name is "ok" whenever it is not empty. Any character is allowed: hyphens, apostrophes, digits, accents and any script, in one part or many.
4. An email is "invalid_email" if it contains a space character (a tab inside does not count), does not have exactly one @, has nothing before or after the @, has no . after the @, or has a domain (the part after the @) that starts or ends with .. Otherwise it is "ok".
5. A phone number is "too_few_digits" when it has fewer than 4 digits from 0 to 9, and "ok" otherwise. Formatting such as +, spaces, brackets and dashes is ignored.