Add vocabulary information text and improve conditional rendering in VocabLessonView

- Introduced a new translation key for vocabulary information text in both English and German, providing context for users on vocabulary usage in exercises.
- Enhanced conditional rendering in VocabLessonView to ensure lesson data is properly checked before displaying cultural notes and vocabulary lists, improving robustness and user experience.

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

@@ -372,6 +372,7 @@
                 "learnVocabulary": "Vokabeln lernen",
                 "culturalNotes": "Kulturelle Notizen",
                 "importantVocab": "Wichtige Begriffe",
+                "vocabInfoText": "Diese Begriffe werden in den Übungen verwendet. Lerne sie hier passiv, bevor du zu den interaktiven Übungen wechselst.",
                 "noVocabInfo": "Lies die Beschreibung oben und die Erklärungen in den Übungen, um die wichtigsten Begriffe zu lernen.",
                 "startExercises": "Zu den Übungen",
                 "correctAnswer": "Richtige Antwort",

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

@@ -372,6 +372,7 @@
                 "learnVocabulary": "Learn Vocabulary",
                 "culturalNotes": "Cultural Notes",
                 "importantVocab": "Important Vocabulary",
+                "vocabInfoText": "These terms are used in the exercises. Learn them here passively before switching to the interactive exercises.",
                 "noVocabInfo": "Read the description above and the explanations in the exercises to learn the most important terms.",
                 "startExercises": "Start Exercises",
                 "correctAnswer": "Correct Answer",