generateUniqueId function
Generates a unique ID based on the current time.
Implementation
String generateUniqueId() {
  const String characters =
      'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  final Random random = Random();
  final String timestamp = (DateTime.now().millisecondsSinceEpoch ~/ 1000)
      .toRadixString(36)
      .padLeft(8, '0');
  String randomPart = '';
  for (int i = 0; i < 20; i++) {
    randomPart += characters[random.nextInt(characters.length)];
  }
  return '$timestamp$randomPart';
}