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/i18n/locales/en/admin.json

@@ -30,6 +30,20 @@
             "actions": "Actions",
             "search": "Search"
         },
+        "vocabLessonReset": {
+            "title": "Language course: lesson progress",
+            "intro": "Delete progress, exercise results and saved lesson state for a single lesson (not the whole course). Only courses you can see as this admin account are listed (public or your own).",
+            "loadCourses": "Load courses",
+            "selectCourse": "Course",
+            "selectLesson": "Lesson",
+            "reset": "Reset lesson for this user",
+            "confirm": "Really delete progress for lesson “{lesson}” for {username}?",
+            "success": "Lesson progress was reset.",
+            "error": "Reset failed.",
+            "pickUserFirst": "Select a user first.",
+            "noCourses": "No courses loaded or no visible courses.",
+            "loadingLessons": "Loading lessons…"
+        },
         "adultVerification": {
             "title": "[Admin] - Erotic approvals",
             "intro": "Adult users can request access to the erotic area. Requests can be reviewed, approved or rejected here.",