Reset exercise answers and results in VocabLessonView before loading new lessons

- Added logic to clear previous exercise answers and results when loading a new lesson, ensuring accurate tracking of user responses.
- Simplified the handling of exercise answers and results by directly assigning values instead of using Vue's `$set` method, improving code readability.

frontend/src/views/social/VocabLessonView.vue

@@ -129,6 +129,9 @@ export default {
   methods: {
     async loadLesson() {
       this.loading = true;
+      // Setze Antworten und Ergebnisse zurück
+      this.exerciseAnswers = {};
+      this.exerciseResults = {};
       try {
         const res = await apiClient.get(`/api/vocab/lessons/${this.lessonId}`);
         this.lesson = res.data;
@@ -154,11 +157,11 @@ export default {
         const exerciseType = this.getExerciseType(exercise);
         if (exerciseType === 'gap_fill') {
           const gapCount = this.getGapCount(exercise);
-          this.$set(this.exerciseAnswers, exercise.id, new Array(gapCount).fill(''));
+          this.exerciseAnswers[exercise.id] = new Array(gapCount).fill('');
         } else {
-          this.$set(this.exerciseAnswers, exercise.id, '');
+          this.exerciseAnswers[exercise.id] = '';
         }
-        this.$set(this.exerciseResults, exercise.id, null);
+        this.exerciseResults[exercise.id] = null;
       });
     },
     async loadGrammarExercises() {
@@ -259,7 +262,7 @@ export default {
         }
 
         const res = await apiClient.post(`/api/vocab/grammar-exercises/${exerciseId}/check`, { answer });
-        this.$set(this.exerciseResults, exerciseId, res.data);
+        this.exerciseResults[exerciseId] = res.data;
       } catch (e) {
         console.error('Fehler beim Prüfen der Antwort:', e);
         alert(e.response?.data?.error || 'Fehler beim Prüfen der Antwort');