secureRandomToken function
Generates a cryptographically secure random token.
length is the number of random bytes and defaults to 32. The bytes come
from Random.secure and are returned as padded URL-safe base64. A negative
length causes List.generate to throw RangeError.
Implementation
String secureRandomToken({int length = 32}) {
final rand = Random.secure();
final bytes = List<int>.generate(length, (_) => rand.nextInt(256));
return base64UrlEncode(bytes);
}