fixed guardian login edge cases and misleading placeholders on failed loads

lib/view/login/guardian_login_controller.dart

@@ -54,9 +54,14 @@ class GuardianLoginController extends ChangeNotifier {
   String? get errorMessage => _errorMessage;
   String? get errorDetails => _errorDetails;
 
-  bool canResend() {
+  bool canResend() => resendCooldown() == Duration.zero;
+
+  /// Restzeit, bis [resend] wieder erlaubt ist.
+  Duration resendCooldown() {
     final pending = _pending;
-    return pending != null && !_now().isBefore(pending.resendAvailableAt);
+    if (pending == null) return Duration.zero;
+    final remaining = pending.resendAvailableAt.difference(_now());
+    return remaining.isNegative ? Duration.zero : remaining;
   }
 
   /// Picks up a request started before the app was closed.
@@ -77,8 +82,12 @@ class GuardianLoginController extends ChangeNotifier {
   Future<bool> requestCode(String email) async {
     final normalized = email.trim().toLowerCase();
     if (DemoMode.matchesGuardian(normalized)) {
-      await _signIn(GuardianSession(email: normalized, isDemo: true));
-      return true;
+      var signedIn = false;
+      await _run(() async {
+        await _signIn(GuardianSession(email: normalized, isDemo: true));
+        signedIn = true;
+      });
+      return signedIn;
     }
     await _run(() async {
       final secret = DeviceBinding.generateSecret();
@@ -128,6 +137,17 @@ class GuardianLoginController extends ChangeNotifier {
     return _complete(linkToken: link.linkToken);
   }
 
+  /// A mail link that does not belong to the server the app talks to (a live
+  /// link while the app points at beta). Without this the tap would do nothing
+  /// at all.
+  void rejectForeignLink() {
+    _errorMessage =
+        'Dieser Anmeldelink gehört zu einem anderen Server als dem, mit dem '
+        'die App gerade verbunden ist. Bitte gib den Code aus der E-Mail ein.';
+    _errorDetails = null;
+    notifyListeners();
+  }
+
   /// Abandons the running request, e.g. to correct a mistyped address.
   Future<void> changeEmail() async {
     await _store.clear();
@@ -140,7 +160,15 @@ class GuardianLoginController extends ChangeNotifier {
 
   Future<bool> _complete({String? code, String? linkToken}) async {
     final pending = _pending;
-    if (pending == null) return false;
+    if (pending == null) {
+      _errorMessage = GuardianLoginException.messageFor(
+        GuardianLoginError.requestExpired,
+      );
+      _errorDetails = null;
+      _step = GuardianLoginStep.enterEmail;
+      notifyListeners();
+      return false;
+    }
     var signedIn = false;
     await _run(() async {
       await _verify.run(
@@ -188,9 +216,16 @@ class GuardianLoginController extends ChangeNotifier {
   }
 
   static Future<void> _defaultSignIn(Session session) async {
-    // Drop any widget snapshot of a previous account before the new one loads.
-    await WidgetSync.clear();
-    await WidgetSync.triggerUpdate();
+    // Sign in first: the one-time code is already spent at this point, so a
+    // failing widget reset must not cost the session (the user would have to
+    // request a fresh mail for a login that actually succeeded).
     await SessionManager().signIn(session);
+    // Drop any widget snapshot of a previous account.
+    try {
+      await WidgetSync.clear();
+      await WidgetSync.triggerUpdate();
+    } on Object catch (e) {
+      log('Guardian login: widget reset failed: $e');
+    }
   }
 }