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.

backend/routers/vocabRouter.js

@@ -0,0 +1,26 @@
+import express from 'express';
+import { authenticate } from '../middleware/authMiddleware.js';
+import VocabController from '../controllers/vocabController.js';
+
+const router = express.Router();
+const vocabController = new VocabController();
+
+router.use(authenticate);
+
+router.get('/languages', vocabController.listLanguages);
+router.post('/languages', vocabController.createLanguage);
+router.post('/subscribe', vocabController.subscribe);
+router.get('/languages/:languageId', vocabController.getLanguage);
+
+// Kapitel
+router.get('/languages/:languageId/chapters', vocabController.listChapters);
+router.post('/languages/:languageId/chapters', vocabController.createChapter);
+router.get('/languages/:languageId/vocabs', vocabController.listLanguageVocabs);
+
+router.get('/chapters/:chapterId', vocabController.getChapter);
+router.get('/chapters/:chapterId/vocabs', vocabController.listChapterVocabs);
+router.post('/chapters/:chapterId/vocabs', vocabController.addVocabToChapter);
+
+export default router;
+
+