Add endpoint to retrieve all available languages in VocabController and VocabRouter

- Introduced a new method in VocabService to list all languages from the database.
- Updated VocabController to wrap the new method for user access.
- Added a new route in VocabRouter to handle requests for all languages.
- Modified VocabCourseListView to utilize the new endpoint for loading languages, enhancing the course selection experience.

frontend/src/views/social/VocabCourseListView.vue

@@ -165,7 +165,8 @@ export default {
   methods: {
     async loadLanguages() {
       try {
-        const res = await apiClient.get('/api/vocab/languages');
+        // Verwende /languages/all für die Kursliste, um alle Sprachen anzuzeigen
+        const res = await apiClient.get('/api/vocab/languages/all');
         this.languages = res.data?.languages || [];
         
         // Lade die Muttersprache des Benutzers