added support for multiple accounts, guardian login bugfixes, ui changes

lib/api/marianumconnect/auth/session_validator.dart

@@ -10,10 +10,10 @@ import '../queries/auth_verify/auth_verify.dart';
 /// Credential probe. For password accounts a server-side password rotation
 /// forces a re-login on the next cold start even when the bearer token would
 /// still be accepted; for guardians it confirms a rejected token before the
-/// session is dropped.
+/// session is dropped. Another stored account then takes over.
 class SessionValidator {
   static Future<void> probeStored({
-    required Future<void> Function() onInvalidated,
+    required Future<void> Function(String? nextAccountId) onInvalidated,
   }) async {
     final session = SessionManager().current;
     // The probes use their own dio (bypassing the demo interceptor), so a demo
@@ -29,7 +29,7 @@ class SessionValidator {
     } on AuthException catch (e) {
       if (e.statusCode != 401) return;
       log('MC: stored session rejected — forcing re-login');
-      await SessionLifecycle.signOut(
+      final next = await SessionLifecycle.signOut(
         notice: switch (session) {
           CredentialSession() =>
             'Deine Zugangsdaten wurden vom Server abgelehnt. Vermutlich '
@@ -38,7 +38,7 @@ class SessionValidator {
             'Deine Anmeldung ist abgelaufen. Bitte melde dich erneut an.',
         },
       );
-      await onInvalidated();
+      await onInvalidated(next);
     } catch (e) {
       log('MC: background session check failed (transient): $e');
     }