feat(vocab): add dashboard learning summary and related endpoints

- Introduced `getDashboardLearningSummary` method in `VocabService` to provide a compact overview of enrolled courses and current lessons for users.
- Updated `vocabController` to include a new route for the dashboard widget, allowing users to access their learning summary.
- Enhanced `vocabRouter` to route requests for the new dashboard widget endpoint.
- Added localization support for the new dashboard features across multiple languages, improving user engagement and accessibility.
- Updated UI components to integrate the new dashboard widget, ensuring a seamless user experience.

frontend/src/i18n/locales/de/general.json

@@ -92,6 +92,16 @@
         },
         "falukant": {
             "emptyValue": "—"
+        },
+        "vocabCourses": {
+            "empty": "Du bist derzeit in keinem Vokabelkurs eingeschrieben.",
+            "browseCourses": "Kurse entdecken",
+            "unnamedCourse": "Kurs ohne Titel",
+            "lessonLine": "Lektion #{number}: {title}",
+            "noLessons": "Noch keine Lektionen in diesem Kurs.",
+            "allDone": "Alle Lektionen absolviert",
+            "openLesson": "Zur Lektion",
+            "openCourse": "Zum Kurs"
         }
     },
     "gender": {