feat(admin): add user vocab course management functionality

- Implemented `getUserVocabCourses` and `getVocabCourseForAdmin` methods in `AdminController` to allow admins to retrieve enrolled vocab courses for users and specific course details, respectively.
- Updated `adminRouter` to include new routes for accessing user vocab courses and course details.
- Enhanced `AdminService` with methods to list user-enrolled vocab courses and retrieve course information with lessons, ensuring proper access control.
- Improved `VocabService` to support the new functionalities, including attaching language names to course data.
- Updated UI components in `UsersView` to reflect changes, including error handling and loading states for course retrieval, along with localization updates for new features.

frontend/src/views/admin/UsersView.vue

@@ -50,12 +50,15 @@
         <span>{{ $t('admin.vocabLessonReset.selectCourse') }}</span>
         <select v-model="selectedVocabCourseId" @change="onVocabCourseChange">
           <option value="">{{ $t('admin.vocabLessonReset.selectCourse') }}</option>
-          <option v-for="c in vocabCourses" :key="c.id" :value="String(c.id)">{{ c.title }}</option>
+          <option v-for="c in vocabCoursesForSelect" :key="c.id" :value="String(c.id)">{{ c.selectLabel }}</option>
         </select>
       </label>
 
+      <p v-else-if="vocabCoursesAttempted && !loadingVocabCourses && vocabLoadError" class="vocab-reset__hint">
+        {{ $t('admin.vocabLessonReset.loadCoursesError') }}
+      </p>
       <p v-else-if="vocabCoursesAttempted && !loadingVocabCourses" class="vocab-reset__hint">
-        {{ $t('admin.vocabLessonReset.noCourses') }}
+        {{ $t('admin.vocabLessonReset.noEnrolledCourses') }}
       </p>
 
       <label v-if="vocabCourseLessons.length" class="edit__field">
@@ -101,13 +104,34 @@ export default {
       selectedVocabLessonId: '',
       loadingVocabCourses: false,
       loadingVocabCourseDetail: false,
-      vocabResetSubmitting: false
+      vocabResetSubmitting: false,
+      vocabLoadError: false
     };
   },
+  computed: {
+    /** Gleicher Titel bei mehreren Kurs-IDs → Kurs-ID anhängen (z. B. geklonte Kurse). */
+    vocabCoursesForSelect() {
+      const list = this.vocabCourses;
+      const counts = {};
+      list.forEach((c) => {
+        const t = (c.title || '').trim() || '—';
+        counts[t] = (counts[t] || 0) + 1;
+      });
+      return list.map((c) => {
+        const t = (c.title || '').trim() || '—';
+        const dup = counts[t] > 1;
+        return {
+          ...c,
+          selectLabel: dup ? `${c.title} (#${c.id})` : (c.title || `#${c.id}`)
+        };
+      });
+    }
+  },
   methods: {
     clearVocabResetUi() {
       this.vocabCourses = [];
       this.vocabCoursesAttempted = false;
+      this.vocabLoadError = false;
       this.selectedVocabCourseId = '';
       this.vocabCourseLessons = [];
       this.selectedVocabLessonId = '';
@@ -137,8 +161,9 @@ export default {
       }
       this.loadingVocabCourses = true;
       this.vocabCoursesAttempted = true;
+      this.vocabLoadError = false;
       try {
-        const { data } = await apiClient.get('/api/vocab/courses');
+        const { data } = await apiClient.get(`/api/admin/users/${this.selected.id}/vocab-courses`);
         this.vocabCourses = Array.isArray(data) ? data : [];
         this.selectedVocabCourseId = '';
         this.vocabCourseLessons = [];
@@ -146,6 +171,7 @@ export default {
       } catch (e) {
         console.error('[UsersView] vocab courses:', e);
         this.vocabCourses = [];
+        this.vocabLoadError = true;
         this.$root?.$refs?.messageDialog?.open?.(this.$t('admin.vocabLessonReset.error'));
       } finally {
         this.loadingVocabCourses = false;
@@ -159,7 +185,7 @@ export default {
       }
       this.loadingVocabCourseDetail = true;
       try {
-        const { data } = await apiClient.get(`/api/vocab/courses/${this.selectedVocabCourseId}`);
+        const { data } = await apiClient.get(`/api/admin/vocab/courses/${this.selectedVocabCourseId}`);
         const lessons = data?.lessons || [];
         this.vocabCourseLessons = [...lessons].sort((a, b) => {
           if (a.weekNumber !== b.weekNumber) {