Enhance VocabLessonView and VocabService with new learning features

- Added a tabbed interface in VocabLessonView for 'Learn' and 'Exercises' sections, improving user navigation.
- Implemented logic to display important vocabulary and cultural notes in the learning section.
- Updated exercise result display to include correct answers and alternatives for better user feedback.
- Enhanced VocabService to extract correct answers and alternatives from exercise data, supporting the new UI features.
- Added new translations for vocabulary-related terms in both English and German, ensuring consistency across the application.

backend/services/vocabService.js

@@ -1310,8 +1310,20 @@ export default class VocabService {
       await progress.save();
     }
 
+    // Extrahiere richtige Antwort und Alternativen
+    const answerData = typeof exercise.answerData === 'string' 
+      ? JSON.parse(exercise.answerData) 
+      : exercise.answerData;
+    
+    const correctAnswer = Array.isArray(answerData.correct) 
+      ? answerData.correct[0] 
+      : answerData.correct;
+    const alternatives = answerData.alternatives || [];
+
     return {
       correct: isCorrect,
+      correctAnswer: correctAnswer,
+      alternatives: alternatives,
       explanation: exercise.explanation,
       progress: progress.get({ plain: true })
     };