replaced account data singleton with a session model

lib/background/widget_background_task.dart

@@ -18,7 +18,8 @@ import '../api/marianumconnect/queries/timetable_get_week/timetable_get_week.dar
 import '../api/mhsl/custom_timetable_event/get/get_custom_timetable_event.dart';
 import '../api/mhsl/custom_timetable_event/get/get_custom_timetable_event_params.dart';
 import '../api/mhsl/custom_timetable_event/get/get_custom_timetable_event_response.dart';
-import '../model/account_data.dart';
+import '../session/session.dart';
+import '../session/session_manager.dart';
 import '../widget_data/widget_data_mapper.dart';
 import '../widget_data/widget_publisher.dart';
 import '../widget_data/widget_sync.dart';
@@ -81,17 +82,17 @@ class WidgetBackgroundTask {
   /// Throws on fetch failure so the worker path can signal a retry.
   static Future<void> runRefreshNow({bool force = false}) async {
     await WidgetSync.ensureInitialized();
-    bool populated;
+    Session? session;
     try {
       // Bounded: a hanging keystore read must not stall the caller's budget
       // (FCM handler ~25s on iOS) forever.
-      populated = await AccountData().waitForPopulation().timeout(
+      session = await SessionManager().waitForLoad().timeout(
         const Duration(seconds: 10),
       );
     } on TimeoutException {
-      populated = false;
+      session = null;
     }
-    if (!populated) {
+    if (session == null) {
       // Deliberately does NOT flip the widget to logged-out: a failed or slow
       // keychain read (locked iOS device during the 06:00 silent push) is
       // indistinguishable from "never logged in" here, and blanking the
@@ -105,7 +106,7 @@ class WidgetBackgroundTask {
       log('[widget-refresh] snapshot is fresh, skipping refresh');
       return;
     }
-    await _refresh();
+    await _refresh(session);
   }
 
   static Future<void> cancelAll() async {
@@ -142,7 +143,7 @@ void _callbackDispatcher() {
   });
 }
 
-Future<void> _refresh() async {
+Future<void> _refresh(Session session) async {
   await WidgetSync.ensureInitialized();
   // The background isolate doesn't go through main.dart's BlocBuilder, so we
   // re-apply the endpoint the foreground last persisted. Without this the
@@ -181,11 +182,14 @@ Future<void> _refresh() async {
   final timegridFuture = _runOrNull<TimetableGetTimegridResponse>(
     () => TimetableGetTimegrid().run(),
   );
-  final customEventsFuture = _runOrNull<GetCustomTimetableEventResponse>(
-    () => GetCustomTimetableEvent(
-      GetCustomTimetableEventParams(AccountData().getUserSecret()),
-    ).run(),
-  );
+  final customEventsFuture = switch (session) {
+    CredentialSession(:final legacyUserSecret) =>
+      _runOrNull<GetCustomTimetableEventResponse>(
+        () => GetCustomTimetableEvent(
+          GetCustomTimetableEventParams(legacyUserSecret),
+        ).run(),
+      ),
+  };
   final timetable = await timetableFuture;
   final subjects = await subjectsFuture;
   final rooms = await roomsFuture;