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/controllers/vocabController.js

@@ -32,6 +32,7 @@ class VocabController {
     this.deleteCourse = this._wrapWithUser((userId, req) => this.service.deleteCourse(userId, req.params.courseId));
     
     // Lessons
+    this.getLesson = this._wrapWithUser((userId, req) => this.service.getLesson(userId, req.params.lessonId));
     this.addLessonToCourse = this._wrapWithUser((userId, req) => this.service.addLessonToCourse(userId, req.params.courseId, req.body), { successStatus: 201 });
     this.updateLesson = this._wrapWithUser((userId, req) => this.service.updateLesson(userId, req.params.lessonId, req.body));
     this.deleteLesson = this._wrapWithUser((userId, req) => this.service.deleteLesson(userId, req.params.lessonId));