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/controllers/vocabController.js

@@ -0,0 +1,45 @@
+import VocabService from '../services/vocabService.js';
+
+function extractHashedUserId(req) {
+  return req.headers?.userid;
+}
+
+class VocabController {
+  constructor() {
+    this.service = new VocabService();
+
+    this.listLanguages = this._wrapWithUser((userId) => this.service.listLanguages(userId));
+    this.createLanguage = this._wrapWithUser((userId, req) => this.service.createLanguage(userId, req.body), { successStatus: 201 });
+    this.subscribe = this._wrapWithUser((userId, req) => this.service.subscribeByShareCode(userId, req.body), { successStatus: 201 });
+    this.getLanguage = this._wrapWithUser((userId, req) => this.service.getLanguage(userId, req.params.languageId));
+
+    this.listChapters = this._wrapWithUser((userId, req) => this.service.listChapters(userId, req.params.languageId));
+    this.createChapter = this._wrapWithUser((userId, req) => this.service.createChapter(userId, req.params.languageId, req.body), { successStatus: 201 });
+    this.listLanguageVocabs = this._wrapWithUser((userId, req) => this.service.listLanguageVocabs(userId, req.params.languageId));
+
+    this.getChapter = this._wrapWithUser((userId, req) => this.service.getChapter(userId, req.params.chapterId));
+    this.listChapterVocabs = this._wrapWithUser((userId, req) => this.service.listChapterVocabs(userId, req.params.chapterId));
+    this.addVocabToChapter = this._wrapWithUser((userId, req) => this.service.addVocabToChapter(userId, req.params.chapterId, req.body), { successStatus: 201 });
+  }
+
+  _wrapWithUser(fn, { successStatus = 200 } = {}) {
+    return async (req, res) => {
+      try {
+        const hashedUserId = extractHashedUserId(req);
+        if (!hashedUserId) {
+          return res.status(400).json({ error: 'Missing user identifier' });
+        }
+        const result = await fn(hashedUserId, req, res);
+        res.status(successStatus).json(result);
+      } catch (error) {
+        console.error('Controller error:', error);
+        const status = error.status && typeof error.status === 'number' ? error.status : 500;
+        res.status(status).json({ error: error.message || 'Internal error' });
+      }
+    };
+  }
+}
+
+export default VocabController;
+
+