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/services/vocabService.js

@@ -846,6 +846,33 @@ export default class VocabService {
     return { success: true };
   }
 
+  async getLesson(hashedUserId, lessonId) {
+    const user = await this._getUserByHashedId(hashedUserId);
+    const lesson = await VocabCourseLesson.findByPk(lessonId, {
+      include: [
+        {
+          model: VocabCourse,
+          as: 'course'
+        }
+      ]
+    });
+
+    if (!lesson) {
+      const err = new Error('Lesson not found');
+      err.status = 404;
+      throw err;
+    }
+
+    // Prüfe Zugriff
+    if (lesson.course.ownerUserId !== user.id && !lesson.course.isPublic) {
+      const err = new Error('Access denied');
+      err.status = 403;
+      throw err;
+    }
+
+    return lesson.get({ plain: true });
+  }
+
   async addLessonToCourse(hashedUserId, courseId, { chapterId, lessonNumber, title, description, weekNumber, dayNumber, lessonType, audioUrl, culturalNotes, targetMinutes, targetScorePercent, requiresReview }) {
     const user = await this._getUserByHashedId(hashedUserId);
     const course = await VocabCourse.findByPk(courseId);