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.

backend/services/vocabService.js

@@ -2485,6 +2485,66 @@ export default class VocabService {
     return this._serializeLessonProgress(progress, lessonData);
   }
 
+  /**
+   * Löscht nur den Fortschritt zu einer Lektion (Zeile vocab_course_progress + zugehörige grammar-exercise-progress).
+   * Gesamtkurs / andere Lektionen bleiben unberührt.
+   */
+  async _purgeLessonProgressForUser(userId, lessonId) {
+    const numericLessonId = Number(lessonId);
+    const exercises = await VocabGrammarExercise.findAll({
+      where: { lessonId: numericLessonId },
+      attributes: ['id']
+    });
+    const exerciseIds = exercises.map((e) => e.id);
+    let deletedExerciseProgressRows = 0;
+    if (exerciseIds.length > 0) {
+      deletedExerciseProgressRows = await VocabGrammarExerciseProgress.destroy({
+        where: { userId, exerciseId: { [Op.in]: exerciseIds } }
+      });
+    }
+    const deletedLessonProgressRows = await VocabCourseProgress.destroy({
+      where: { userId, lessonId: numericLessonId }
+    });
+    return {
+      success: true,
+      lessonId: numericLessonId,
+      deletedLessonProgressRows,
+      deletedExerciseProgressRows
+    };
+  }
+
+  /** Eingeloggter Nutzer setzt eigene Lektion zurück (nur bei Kurs-Einschreibung). */
+  async resetMyLessonProgress(hashedUserId, lessonId) {
+    const user = await this._getUserByHashedId(hashedUserId);
+    const lesson = await VocabCourseLesson.findByPk(Number(lessonId));
+    if (!lesson) {
+      const err = new Error('Lesson not found');
+      err.status = 404;
+      throw err;
+    }
+    const enrollment = await VocabCourseEnrollment.findOne({
+      where: { userId: user.id, courseId: lesson.courseId }
+    });
+    if (!enrollment) {
+      const err = new Error('Not enrolled in this course');
+      err.status = 403;
+      throw err;
+    }
+    return this._purgeLessonProgressForUser(user.id, lesson.id);
+  }
+
+  /** Admin: Zielnutzer per Hash, ohne Einschreibungszwang (idempotentes Löschen). */
+  async adminResetLessonProgressForUser(targetHashedUserId, lessonId) {
+    const user = await this._getUserByHashedId(targetHashedUserId);
+    const lesson = await VocabCourseLesson.findByPk(Number(lessonId));
+    if (!lesson) {
+      const err = new Error('Lesson not found');
+      err.status = 404;
+      throw err;
+    }
+    return this._purgeLessonProgressForUser(user.id, lesson.id);
+  }
+
   // ========== GRAMMAR EXERCISE METHODS ==========
 
   async getExerciseTypes() {