Enhance VocabLessonView and VocabService for grammar exercise handling

- Added logic to initialize grammar exercises directly from lesson data or load them separately if not included.
- Introduced a new method to initialize answer arrays for gap fill exercises, improving user interaction and response tracking.
- Updated comments for clarity on the exercise loading process and initialization logic.

backend/services/vocabService.js

@@ -853,6 +853,19 @@ export default class VocabService {
         {
           model: VocabCourse,
           as: 'course'
+        },
+        {
+          model: VocabGrammarExercise,
+          as: 'grammarExercises',
+          include: [
+            {
+              model: VocabGrammarExerciseType,
+              as: 'exerciseType'
+            }
+          ],
+          required: false,
+          separate: true,
+          order: [['exerciseNumber', 'ASC']]
         }
       ]
     });