fix(auth_service): add savedFilters to default user preferences

- Add savedFilters to the list of followedSources, followedTopics, and savedHeadlines in the defaultUserPreferences object
- This ensures that new user preferences will have an empty list for savedFilters by default

Author: fulleni

lib/src/services/auth_service.dart

@@ -564,6 +564,7 @@ class AuthService {
         followedSources: const [],
         followedTopics: const [],
         savedHeadlines: const [],
+        savedFilters: const [],
       );
       await _userContentPreferencesRepository.create(
         item: defaultUserPreferences,