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/views/home/LoggedInView.vue

@@ -192,7 +192,8 @@ export default {
         '/api/news': 'home.dashboard.widgetLabels.news',
         '/api/calendar/widget/birthdays': 'home.dashboard.widgetLabels.birthdays',
         '/api/calendar/widget/upcoming': 'home.dashboard.widgetLabels.upcoming',
-        '/api/calendar/widget/mini': 'home.dashboard.widgetLabels.calendar'
+        '/api/calendar/widget/mini': 'home.dashboard.widgetLabels.calendar',
+        '/api/vocab/dashboard-widget': 'home.dashboard.widgetLabels.vocabCourses'
       }[endpoint];
       return key ? this.$t(key) : fallbackLabel;
     },
@@ -211,7 +212,8 @@ export default {
         'News',
         'Geburtstage',
         'Nächste Termine',
-        'Kalender'
+        'Kalender',
+        'Sprachkurse'
       ];
       return {
         ...widget,