fix(learn): an editor with nobody signed in above it checks nothing

The full window editor is also shown without a person's scope, as a
privacy test pumps it, where GrammarHost asserted for want of one.
halkyon_app's PersonScope gains maybeOf beside of.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/app/person_scope_test.dart

@@ -45,6 +45,23 @@ void main() {
     expect(find.text('Data of ada'), findsOneWidget);
   });
 
+  testWidgets('maybeOf finds the dependencies of the person signed in '
+      'above, and none where nobody is', (tester) async {
+    TestPerson? found;
+    Widget looking() => Builder(
+      builder: (context) {
+        found = PersonScope.maybeOf<TestPerson>(context);
+        return const SizedBox();
+      },
+    );
+
+    await show(tester, PersonScope(person: ada, child: looking()));
+    expect(found?.person, ada);
+
+    await show(tester, looking());
+    expect(found, isNull);
+  });
+
   testWidgets("closes the person's data when it goes away", (tester) async {
     await show(tester, const PersonScope(person: ada, child: _Person()));