replaced account data singleton with a session model

lib/api/marianumconnect/auth/auth_interceptor.dart

@@ -1,12 +1,13 @@
 import 'package:dio/dio.dart';
 
-import '../../../model/account_data.dart';
+import '../../../session/session.dart';
+import '../../../session/session_manager.dart';
 import '../queries/auth_login/auth_login.dart';
 import 'device_token_name.dart';
 import 'token_storage.dart';
 
 /// Adds the bearer token to outgoing Marianum-Connect requests and, on 401,
-/// re-logs in once with the credentials in [AccountData] before retrying.
+/// renews the token once before retrying.
 class MarianumConnectAuthInterceptor extends Interceptor {
   static const _retriedKey = 'mc_auth_retried';
 
@@ -87,25 +88,26 @@ class MarianumConnectAuthInterceptor extends Interceptor {
   }
 
   Future<bool> _performReLogin() async {
-    if (!AccountData().isPopulated()) return false;
-    final username = AccountData().getUsername();
+    final session = SessionManager().current;
+    if (session is! CredentialSession) return false;
+    final username = session.username;
     // A background engine (widget task) keeps the account it loaded. When
     // the app signed that account out, its revoked token answers 401 — a
     // re-login would mint a fresh token for it into the shared keystore.
-    if (await AccountData().readStoredUsername() != username) return false;
+    if (await SessionManager().readStoredUsername() != username) return false;
     try {
       await _loginClient.run(
         username: username,
-        password: AccountData().getPassword(),
+        password: session.password,
         tokenName: await DeviceTokenName.resolve(),
       );
     } catch (_) {
-      if (await AccountData().readStoredUsername() == username) {
+      if (await SessionManager().readStoredUsername() == username) {
         await _tokenStorage.clear();
       }
       return false;
     }
-    final stored = await AccountData().readStoredUsername();
+    final stored = await SessionManager().readStoredUsername();
     if (stored == username) return true;
     // Signed out during the login: drop the orphaned token, unless another
     // account already stored its own.