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.

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

@@ -366,7 +366,16 @@
                 "checkAnswer": "Antwort prüfen",
                 "correct": "Richtig!",
                 "wrong": "Falsch",
-                "explanation": "Erklärung"
+                "explanation": "Erklärung",
+                "learn": "Lernen",
+                "exercises": "Übungen",
+                "learnVocabulary": "Vokabeln lernen",
+                "culturalNotes": "Kulturelle Notizen",
+                "importantVocab": "Wichtige Begriffe",
+                "noVocabInfo": "Lies die Beschreibung oben und die Erklärungen in den Übungen, um die wichtigsten Begriffe zu lernen.",
+                "startExercises": "Zu den Übungen",
+                "correctAnswer": "Richtige Antwort",
+                "alternatives": "Alternative Antworten"
             }
         }
     }

frontend/src/i18n/locales/en/socialnetwork.json

@@ -366,7 +366,16 @@
                 "checkAnswer": "Check Answer",
                 "correct": "Correct!",
                 "wrong": "Wrong",
-                "explanation": "Explanation"
+                "explanation": "Explanation",
+                "learn": "Learn",
+                "exercises": "Exercises",
+                "learnVocabulary": "Learn Vocabulary",
+                "culturalNotes": "Cultural Notes",
+                "importantVocab": "Important Vocabulary",
+                "noVocabInfo": "Read the description above and the explanations in the exercises to learn the most important terms.",
+                "startExercises": "Start Exercises",
+                "correctAnswer": "Correct Answer",
+                "alternatives": "Alternative Answers"
             }
         }
     }