getUpgradeErrorMessage method
Maps a FirebaseAuthException code from an upgrade attempt to a
user-friendly message.
Implementation
String getUpgradeErrorMessage(FirebaseAuthException e) => switch (e.code) {
'email-already-in-use' => 'This email is already registered',
'invalid-email' => 'Please enter a valid email address',
'weak-password' => 'Password should be at least 6 characters',
'requires-recent-login' => 'Session expired. Please sign in again',
_ => 'Account upgrade failed: ${e.message}',
};