replaced account data singleton with a session model

lib/utils/session_single_flight.dart

@@ -1,4 +1,4 @@
-import '../model/account_data.dart';
+import '../session/session_manager.dart';
 
 /// Joins concurrent calls into the one already running for the same account
 /// session. A run left over from a signed-out session never blocks the next
@@ -8,7 +8,7 @@ class SessionSingleFlight {
   int? _epoch;
 
   Future<void> run(Future<void> Function() action) {
-    final epoch = AccountData().sessionEpoch;
+    final epoch = SessionManager().sessionEpoch;
     final running = _running;
     if (running != null && _epoch == epoch) return running;
     _epoch = epoch;