added guardian letters with chat and multiple answer functionalities

lib/push/push_target.dart

@@ -0,0 +1,45 @@
+/// `type` of the visible Connect push for a parent letter. Its data carries
+/// the letter under [parentLetterIdKey].
+const String parentLetterPushType = 'parent-letter';
+const String parentLetterIdKey = 'parentLetterId';
+
+/// Where a tapped notification leads.
+sealed class PushTarget {
+  const PushTarget();
+}
+
+class ParentLetterTarget extends PushTarget {
+  final String letterId;
+  const ParentLetterTarget(this.letterId);
+}
+
+class NewsletterTarget extends PushTarget {
+  final String newsletterId;
+  const NewsletterTarget(this.newsletterId);
+}
+
+class ChatTarget extends PushTarget {
+  final String chatToken;
+  const ChatTarget(this.chatToken);
+}
+
+/// Resolves the data of a tapped notification — the payload of a locally
+/// rendered one as well as the data of an FCM message. Null when it names no
+/// known target.
+PushTarget? resolvePushTarget(Map<String, dynamic> data) {
+  String? value(String key) {
+    final value = data[key];
+    return value is String && value.isNotEmpty ? value : null;
+  }
+
+  if (value(parentLetterIdKey) case final letterId?) {
+    return ParentLetterTarget(letterId);
+  }
+  if (value('newsletterId') case final newsletterId?) {
+    return NewsletterTarget(newsletterId);
+  }
+  for (final key in const ['chatToken', 'token', 'roomToken']) {
+    if (value(key) case final chatToken?) return ChatTarget(chatToken);
+  }
+  return null;
+}