added support for multiple accounts with an account switcher in settings

lib/api/marianumconnect/auth/session_validator.dart

@@ -8,9 +8,10 @@ import '../queries/auth_verify/auth_verify.dart';
 
 /// Credential probe. A server-side password rotation forces a re-login on the
 /// next cold start even when the bearer token would still be accepted.
+/// 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
@@ -24,18 +25,18 @@ class SessionValidator {
       }
     } on AuthException catch (e) {
       if (e.statusCode != 401) return;
-      // The probed account already signed out; the 401 must not sign out
-      // whoever logged in meanwhile.
+      // The probed account is no longer the active one; the 401 must not
+      // sign out whoever took over meanwhile.
       if (!SessionManager().isCurrentSession(epoch)) 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 '
                 'wurde dein Passwort geändert. Bitte melde dich erneut an.',
         },
       );
-      await onInvalidated();
+      await onInvalidated(next);
     } catch (e) {
       log('MC: background session check failed (transient): $e');
     }