Fix: Null-safe access to rightType.title in menu endpoint

- Verwende Optional Chaining (?.) für ur.rightType.title
- Filtere null/undefined Werte aus rights Array
- Behebt: Cannot read properties of null (reading 'title')

backend/controllers/navigationController.js

@@ -359,7 +359,7 @@ class NavigationController {
             });
             const birthDate = userBirthdateParams.length > 0 ? userBirthdateParams[0].value : (new Date()).toDateString();
             const age = this.calculateAge(birthDate);
-            const rights = userRights.map(ur => ur.rightType.title);
+            const rights = userRights.map(ur => ur.rightType?.title).filter(Boolean);
             const filteredMenu = await this.filterMenu(menuStructure, rights, age, user.id);
             res.status(200).json(filteredMenu);
         } catch (error) {