Add heir management functionality in Falukant module

- Implemented setHeir method in FalukantService to designate a child as heir, including validation checks for user and child relationships.
- Updated FalukantController to expose the setHeir endpoint, allowing users to set heirs via the API.
- Enhanced FalukantRouter with a new route for setting heirs.
- Modified FamilyView component to include UI elements for setting heirs, with success and error feedback.
- Updated localization files in both German and English to include new translations related to heir management, improving user experience.

frontend/src/i18n/locales/de/falukant.json

@@ -405,6 +405,11 @@
                 "title": "Kinder",
                 "name": "Name",
                 "age": "Alter",
+                "heir": "Erbe",
+                "isHeir": "Erbe",
+                "setAsHeir": "Als Erben festlegen",
+                "heirSetSuccess": "Das Kind wurde erfolgreich als Erbe festgelegt.",
+                "heirSetError": "Fehler beim Festlegen des Erben.",
                 "actions": "Aktionen",
                 "none": "Keine Kinder vorhanden.",
                 "detailButton": "Details anzeigen",