Add lesson retrieval functionality in VocabController and VocabService

- Introduced a new method in VocabService to fetch lesson details, including access control based on user ownership and lesson visibility.
- Updated VocabController to wrap the new method for user access.
- Added a new route in VocabRouter to handle requests for specific lessons.
- Enhanced VocabCourseListView to support navigation to individual lesson views, improving user experience in accessing lesson content.

backend/routers/vocabRouter.js

@@ -43,6 +43,7 @@ router.delete('/courses/:courseId/enroll', vocabController.unenrollFromCourse);
 
 // Progress
 router.get('/courses/:courseId/progress', vocabController.getCourseProgress);
+router.get('/lessons/:lessonId', vocabController.getLesson);
 router.put('/lessons/:lessonId/progress', vocabController.updateLessonProgress);
 
 // Grammar Exercises