Add Vocab Trainer feature with routing, database schema, and translations

- Introduced Vocab Trainer functionality, including new routes for managing languages and chapters.
- Implemented database schema for vocab-related tables to ensure data integrity.
- Updated navigation and UI components to include Vocab Trainer in the social network menu.
- Added translations for Vocab Trainer in both German and English locales, enhancing user accessibility.

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

@@ -26,7 +26,10 @@
         }
     },
     "general": {
-        "datetimelong": "dd.MM.yyyy HH:mm:ss"
+        "datetimelong": "dd.MM.yyyy HH:mm:ss",
+        "loading": "Lädt...",
+        "back": "Zurück",
+        "cancel": "Abbrechen"
     },
     "OK": "Ok",
     "Cancel": "Abbrechen",