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/i18n/locales/en/admin.json

@@ -32,8 +32,8 @@
         },
         "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",
+            "intro": "Delete progress, exercise results and saved lesson state for a single lesson (not the whole course). Only language courses this user is enrolled in are listed.",
+            "loadCourses": "Load enrolled courses",
             "selectCourse": "Course",
             "selectLesson": "Lesson",
             "reset": "Reset lesson for this user",
@@ -41,7 +41,8 @@
             "success": "Lesson progress was reset.",
             "error": "Reset failed.",
             "pickUserFirst": "Select a user first.",
-            "noCourses": "No courses loaded or no visible courses.",
+            "noEnrolledCourses": "This user is not enrolled in any language course.",
+            "loadCoursesError": "Could not load the course list.",
             "loadingLessons": "Loading lessons…"
         },
         "adultVerification": {