getMasteryIntroSeen method
One-time flag for the mastery-ladder intro banner shown at the top of
TopicTreeScreen. The banner replaces the standalone
/onboarding/mastery screen — the explanation only surfaces when the
user actually opens a domain's topic tree and goes away after dismissal.
Implementation
Future<bool> getMasteryIntroSeen() async {
final p = await _prefs();
return p.getBool(_masteryIntroSeenKey) ?? false;
}