feat(vocab): enhance lesson state management and persistence in VocabLessonView

- Added a new JSONB field `lessonState` to the VocabCourseProgress model to store detailed lesson state information.
- Implemented methods in VocabService for sanitizing and serializing lesson state, ensuring robust data handling.
- Updated VocabLessonView to manage lesson state persistence, including local storage and server synchronization, improving user experience during vocabulary lessons.
- Introduced mechanisms for exporting and normalizing exercise answers, enhancing the accuracy of saved progress.

backend/models/community/vocab_course_progress.js

@@ -34,6 +34,12 @@ VocabCourseProgress.init({
         allowNull: false,
         defaultValue: 0
     },
+    lessonState: {
+        type: DataTypes.JSONB,
+        allowNull: false,
+        defaultValue: {},
+        field: 'lesson_state'
+    },
     lastAccessedAt: {
         type: DataTypes.DATE,
         allowNull: true,