Integrate email notifications into signup and login (#38)

frontend/app/lib/auth/feedback.dart

@@ -19,6 +19,7 @@ class AuthFeedback extends StatelessWidget {
               ? (signUp ? l.signupPreview : l.loginPreview)
               : l.authCompleted)
         : switch (error) {
+            AuthError.accountNotVerified => l.authVerifyEmail,
             AuthError.invalidCredentials => l.authInvalidCredentials,
             AuthError.emailInUse => l.authEmailInUse,
             AuthError.unavailable => l.authUnavailable,