feat: increase max email headers from 10 to 30

Real-world emails commonly exceed 10 headers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: aterga

src/internet_identity_interface/src/internet_identity/types/smtp.rs

@@ -7,7 +7,7 @@ pub const MAX_EMAIL_USER_BYTES: usize = 64;
 pub const MAX_EMAIL_DOMAIN_BYTES: usize = 255;
 pub const MAX_SUBJECT_BYTES: usize = 256;
 pub const MAX_BODY_BYTES: usize = 5_000;
-pub const MAX_HEADERS: usize = 10;
+pub const MAX_HEADERS: usize = 30;
 pub const MAX_HEADER_NAME_BYTES: usize = 256;
 pub const MAX_HEADER_VALUE_BYTES: usize = 8_192;
 pub const MAX_EMAILS_PER_USER: usize = 10;