added guardian letters with chat and multiple answer functionalities

lib/access/access_requirement.dart

@@ -3,10 +3,12 @@ import '../session/session.dart';
 /// Backend identity a feature needs. Modules and settings sections declare
 /// these; anything whose requirements the session does not meet is hidden.
 enum AccessRequirement {
-  nextcloud;
+  nextcloud,
+  guardian;
 
   bool isMetBy(Session? session) => switch (this) {
     AccessRequirement.nextcloud => session?.nextcloud != null,
+    AccessRequirement.guardian => session is GuardianSession,
   };
 }