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.

frontend/src/views/social/VocabCourseView.vue

@@ -341,6 +341,7 @@ export default {
       progress: [],
       chapters: [],
       srsDueItems: [],
+      srsDueTotal: 0,
       srsLoading: false,
       showAddLessonDialog: false,
       assistantSettings: null,
@@ -384,6 +385,9 @@ export default {
       return this.currentLesson?.pedagogy?.blockNumber || null;
     },
     srsDueCount() {
+      if (Number.isFinite(Number(this.srsDueTotal)) && Number(this.srsDueTotal) > 0) {
+        return Number(this.srsDueTotal);
+      }
       return Array.isArray(this.srsDueItems) ? this.srsDueItems.length : 0;
     },
     dueReviewLessons() {
@@ -553,9 +557,11 @@ export default {
           params: { limit: 40 }
         });
         this.srsDueItems = Array.isArray(data?.items) ? data.items : [];
+        this.srsDueTotal = Number.isFinite(Number(data?.totalDueCount)) ? Number(data.totalDueCount) : this.srsDueItems.length;
       } catch (e) {
         console.warn('Konnte SRS-Fälligkeiten nicht laden:', e);
         this.srsDueItems = [];
+        this.srsDueTotal = 0;
       } finally {
         this.srsLoading = false;
       }