fix(core): ignore surrounding whitespace in the email validator

1. The email check rejected addresses with spaces around them, while the url and required checks already accept them.
2. The email check now ignores leading and trailing whitespace, and the validation guide says so.
3. Replaces #351.

Co-authored by @kevin9327, based on #351.

Before

An email field typed as " a@b.co " shows "Invalid email".

https://github.com/user-attachments/assets/34d44d00-6db6-4ed2-90f7-947ea4db7b6c

After

The same input is valid; an address with a space inside is still rejected.

https://github.com/user-attachments/assets/7a01ab3a-9a71-4716-b8c5-cf2030110c3e