Apply the trailing-comma lint the analyzer asks for

Formatting only, from dart fix. No behaviour change — the release APK and the
full suite both pass either way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/unit/pos_session_test.dart

@@ -164,7 +164,7 @@ void main() {
           baseUrl: 'https://example.invalid/pos',
           client: MockClient(
             (_) async => http.Response(jsonEncode(body), status,
-                headers: {'content-type': 'application/json'}),
+                headers: {'content-type': 'application/json'},),
           ),
         );