added guardian login with views for their assigned childs

lib/access/access_requirement.dart

@@ -0,0 +1,15 @@
+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;
+
+  bool isMetBy(Session? session) => switch (this) {
+    AccessRequirement.nextcloud => session?.nextcloud != null,
+  };
+}
+
+extension AccessRequirements on Set<AccessRequirement> {
+  bool areMetBy(Session? session) => every((r) => r.isMetBy(session));
+}