seenNotifier method
ValueNotifier that flips to true once markSeen fires for tipId.
Listenable UIs (e.g. a "New" badge) can subscribe to auto-clear without
explicit wiring.
Implementation
ValueNotifier<bool> seenNotifier(String tipId) {
return _seenNotifiers.putIfAbsent(
tipId,
() => ValueNotifier<bool>(hasSeen(tipId)),
);
}