feat(vocab): add vocab distractor pool functionality

- Implemented a new endpoint to retrieve a pool of distractors for vocabulary exercises based on prior lessons.
- Updated the VocabController and VocabRouter to include the new getVocabDistractorPool method.
- Enhanced VocabService to classify questions and gather distractors from previous lessons.
- Modified VocabLessonView to fetch and utilize the distractor pool for multiple-choice exercises, improving the learning experience.

backend/routers/vocabRouter.js

@@ -28,6 +28,7 @@ router.post('/courses', vocabController.createCourse);
 router.get('/courses', vocabController.getCourses);
 router.get('/courses/my', vocabController.getMyCourses);
 router.post('/courses/find-by-code', vocabController.getCourseByShareCode);
+router.get('/courses/:courseId/distractor-pool', vocabController.getVocabDistractorPool);
 router.get('/courses/:courseId', vocabController.getCourse);
 router.put('/courses/:courseId', vocabController.updateCourse);
 router.delete('/courses/:courseId', vocabController.deleteCourse);