fixed guardian login edge cases and misleading placeholders on failed loads

lib/view/login/widgets/guardian_login_card.dart

@@ -158,7 +158,7 @@ class _GuardianLoginCardState extends State<GuardianLoginCard> {
   Widget _buildCodeStep(BuildContext context) {
     final theme = Theme.of(context);
     final pending = _controller.pending!;
-    final remaining = pending.resendAvailableAt.difference(DateTime.now());
+    final remaining = _controller.resendCooldown();
     return Form(
       key: _codeFormKey,
       child: LoginCardFrame(