Full-width email addresses slipped past signup checks; fix normalizes input with NFKC

A developer at RoamProxy found that a verification email bounced with a 422 error because the address used full-width characters, such as ｅｘａｍｐｌｅ.tech, instead of standard ASCII. The signup form and database accepted the address, so the mismatch only surfaced when the mail provider rejected it. The team now applies Unicode NFKC normalization at the input boundary to fold compatibility characters into plain ASCII, and rejects addresses that remain non-ASCII. The change covers both email signup and Google sign-in, since both pass through the same function. Non-ASCII addresses such as user@bücher.de are now rejected, because the mail provider cannot deliver to them anyway.