Enhance logging and conditional rendering in VocabService and VocabLessonView

- Added detailed logging in VocabService for lesson retrieval, including lesson ID, title, and exercise count.
- Improved conditional rendering in VocabLessonView to handle cases where grammar exercises may not be present, enhancing user experience.
- Updated logging in VocabLessonView to provide insights into loaded lessons and exercises, aiding in debugging and monitoring.

backend/services/vocabService.js

@@ -883,7 +883,14 @@ export default class VocabService {
       throw err;
     }
 
-    return lesson.get({ plain: true });
+    const plainLesson = lesson.get({ plain: true });
+    console.log(`[getLesson] Lektion ${lessonId} geladen:`, {
+      id: plainLesson.id,
+      title: plainLesson.title,
+      exerciseCount: plainLesson.grammarExercises ? plainLesson.grammarExercises.length : 0,
+      exercises: plainLesson.grammarExercises
+    });
+    return plainLesson;
   }
 
   async addLessonToCourse(hashedUserId, courseId, { chapterId, lessonNumber, title, description, weekNumber, dayNumber, lessonType, audioUrl, culturalNotes, targetMinutes, targetScorePercent, requiresReview }) {