feat(vocab): implement user vocab lesson progress reset functionality

- Added `resetUserVocabLessonProgress` method in `AdminController` to allow admins to reset a user's progress for a specific vocab lesson.
- Introduced corresponding route in `adminRouter` for the new reset functionality.
- Enhanced `VocabService` with methods to purge lesson progress for users, ensuring that only the specified lesson's progress is affected.
- Updated UI components in `UsersView` to facilitate the selection of courses and lessons for resetting progress, including confirmation dialogs and loading states.
- Added localization support for the new reset functionality across multiple languages.
- Implemented reset functionality in `VocabLessonView` for users to reset their own lesson progress.

frontend/src/views/social/VocabLessonView.vue

@@ -5,6 +5,14 @@
       <div class="lesson-header">
         <button @click="back" class="btn-back">{{ $t('general.back') }}</button>
         <h2>{{ lesson.title }}</h2>
+        <button
+          type="button"
+          class="btn-reset-lesson"
+          :disabled="resettingLessonProgress"
+          @click="confirmResetLessonProgress"
+        >
+          {{ resettingLessonProgress ? $t('general.loading') : $t('socialnetwork.vocab.courses.resetLessonProgress') }}
+        </button>
       </div>
       
       <!-- Tabs für Lernen und Übungen -->
@@ -942,7 +950,8 @@ export default {
       lessonStatePersistenceReady: false,
       lessonStateSaveTimer: null,
       lessonStateSaveInFlight: false,
-      pendingLessonStatePayload: null
+      pendingLessonStatePayload: null,
+      resettingLessonProgress: false
     };
   },
   computed: {
@@ -1373,6 +1382,44 @@ export default {
       const userId = this.user?.id || 'guest';
       return `vocab-lesson-state:${LESSON_STATE_VERSION}:${userId}:${this.courseId}:${this.lessonId}`;
     },
+    clearLocalLessonStateCache() {
+      const storageKey = this.getLessonStateStorageKey();
+      if (!storageKey) {
+        return;
+      }
+      try {
+        window.localStorage.removeItem(storageKey);
+      } catch (error) {
+        console.warn('[VocabLessonView] Lokaler Lektions-Cache konnte nicht gelöscht werden:', error);
+      }
+    },
+    confirmResetLessonProgress() {
+      if (!this.lessonId || this.resettingLessonProgress) {
+        return;
+      }
+      if (!window.confirm(this.$t('socialnetwork.vocab.courses.resetLessonProgressConfirm'))) {
+        return;
+      }
+      this.resetLessonProgressOnServer();
+    },
+    async resetLessonProgressOnServer() {
+      if (!this.lessonId || this.resettingLessonProgress) {
+        return;
+      }
+      this.resettingLessonProgress = true;
+      try {
+        await apiClient.delete(`/api/vocab/lessons/${this.lessonId}/progress`);
+        this.clearLocalLessonStateCache();
+        this.$root?.$refs?.messageDialog?.open?.('tr:socialnetwork.vocab.courses.resetLessonProgressSuccess');
+        await this.loadLesson();
+      } catch (e) {
+        console.error('[VocabLessonView] Lektion zurücksetzen fehlgeschlagen:', e);
+        const msg = e?.response?.data?.error || this.$t('socialnetwork.vocab.courses.resetLessonProgressError');
+        this.$root?.$refs?.messageDialog?.open?.(msg);
+      } finally {
+        this.resettingLessonProgress = false;
+      }
+    },
     buildPersistedLessonState() {
       return {
         version: LESSON_STATE_VERSION,
@@ -3104,6 +3151,29 @@ export default {
   margin-bottom: 20px;
 }
 
+.lesson-header h2 {
+  flex: 1;
+  min-width: 0;
+  margin: 0;
+}
+
+.btn-reset-lesson {
+  flex-shrink: 0;
+  padding: 8px 14px;
+  border: 1px solid rgba(140, 90, 60, 0.35);
+  border-radius: 4px;
+  background: rgba(255, 248, 240, 0.95);
+  color: #6b4420;
+  cursor: pointer;
+  font-size: 0.875rem;
+  font-weight: 600;
+}
+
+.btn-reset-lesson:disabled {
+  opacity: 0.65;
+  cursor: not-allowed;
+}
+
 .lesson-overview-card {
   display: flex;
   flex-direction: column;