feat(VocabService, VocabCourseView): enhance SRS item retrieval and UI integration

- Updated VocabService to calculate total due count for spaced repetition system (SRS) items, improving data handling for user queries.
- Modified VocabCourseView to incorporate total due count in the UI, ensuring accurate display of SRS item statistics.
- Enhanced error handling and data validation for SRS due items, improving overall reliability and user experience.

backend/services/vocabService.js

@@ -1801,14 +1801,16 @@ export default class VocabService {
 
     const limit = this._clampInteger(query?.limit, { min: 1, max: 100, fallback: 30 });
     const now = new Date();
+    const dueWhere = {
+      userId: user.id,
+      courseId: Number(course.id),
+      nextDueAt: {
+        [Op.lte]: now
+      }
+    };
+    const totalDueCount = await VocabSrsItem.count({ where: dueWhere });
     const rows = await VocabSrsItem.findAll({
-      where: {
-        userId: user.id,
-        courseId: Number(course.id),
-        nextDueAt: {
-          [Op.lte]: now
-        }
-      },
+      where: dueWhere,
       order: [
         ['nextDueAt', 'ASC'],
         ['wrongCount', 'DESC'],
@@ -1821,6 +1823,8 @@ export default class VocabService {
       courseId: course.id,
       dueAt: now.toISOString(),
       count: rows.length,
+      totalDueCount,
+      limit,
       items: rows.map((item) => ({
         itemKey: item.itemKey,
         courseId: item.courseId,