Keep web account sessions in memory only

SovereignAI-Starter/flutter_app/test/session_token_store_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_app/core/auth/session_token_store.dart';
+
+void main() {
+  group('EphemeralSessionTokenStore', () {
+    test('keeps the token available only in the running app instance', () async {
+      final store = EphemeralSessionTokenStore();
+
+      expect(await store.read(), isNull);
+      await store.write('test-session-token');
+      expect(await store.read(), 'test-session-token');
+    });
+
+    test('deletes the in-memory token on logout', () async {
+      final store = EphemeralSessionTokenStore();
+      await store.write('test-session-token');
+
+      await store.delete();
+
+      expect(await store.read(), isNull);
+    });
+  });
+}