added guardian letters with chat and multiple answer functionalities

test/state/app_modules_order_test.dart

@@ -29,9 +29,15 @@ void main() {
 
       final effective = AppModule.effectiveModuleOrder(settingsWith(custom));
 
+      // parentLetters and ticker are both missing and follow timetable in the
+      // declared order, so they line up behind it.
+      expect(
+        effective.indexOf(Modules.parentLetters),
+        effective.indexOf(Modules.timetable) + 1,
+      );
       expect(
         effective.indexOf(Modules.ticker),
-        effective.indexOf(Modules.timetable) + 1,
+        effective.indexOf(Modules.parentLetters) + 1,
       );
       expect(effective.toSet(), Modules.values.toSet());
     });
@@ -94,10 +100,11 @@ void main() {
     final student = CredentialSession(username: 'max', password: 'pw');
     const guardian = GuardianSession(email: 'e@x.de');
 
-    test('password accounts see every module', () {
-      for (final m in Modules.values) {
-        expect(AppModule.isAvailableFor(m, student), isTrue, reason: m.name);
-      }
+    test('password accounts see every module but the guardian ones', () {
+      final hidden = Modules.values
+          .where((m) => !AppModule.isAvailableFor(m, student))
+          .toSet();
+      expect(hidden, {Modules.parentLetters});
     });
 
     test('guardians lose exactly the Nextcloud modules', () {