jwt static method
Builds an unsigned JWT (header.payload.signature) whose payload carries
claims. The signature is a fixed placeholder — the client only reads
claims, never verifies here.
Implementation
static String jwt(Map<String, Object?> claims) {
String seg(Map<String, Object?> value) =>
base64Url.encode(utf8.encode(jsonEncode(value))).replaceAll('=', '');
final String header = seg(<String, Object?>{'alg': 'none', 'typ': 'JWT'});
final String payload = seg(claims);
return '$header.$payload.sig';
}