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.

backend/services/vocabService.js

@@ -2636,6 +2636,111 @@ export default class VocabService {
     }));
   }
 
+  /**
+   * Kompakte Übersicht für das Start-Dashboard: eingeschriebene Kurse und „aktuelle“ Lektion
+   * (gleiche Logik wie VocabCourseView.currentLesson: erste unvollständige, sonst letzte).
+   */
+  async getDashboardLearningSummary(hashedUserId) {
+    const user = await this._getUserByHashedId(hashedUserId);
+
+    const enrollments = await VocabCourseEnrollment.findAll({
+      where: { userId: user.id },
+      include: [
+        {
+          model: VocabCourse,
+          as: 'course',
+          required: true,
+          attributes: ['id', 'title']
+        }
+      ],
+      order: [['enrolledAt', 'DESC']]
+    });
+
+    const courseById = new Map();
+    for (const e of enrollments) {
+      const c = e.course?.get({ plain: true });
+      if (!c?.id || courseById.has(c.id)) {
+        continue;
+      }
+      courseById.set(c.id, { id: c.id, title: c.title || '' });
+    }
+
+    const coursesMeta = [...courseById.values()];
+    if (coursesMeta.length === 0) {
+      return { courses: [] };
+    }
+
+    const courseIds = coursesMeta.map((c) => c.id);
+
+    const lessons = await VocabCourseLesson.findAll({
+      where: { courseId: { [Op.in]: courseIds } },
+      attributes: ['id', 'courseId', 'lessonNumber', 'title'],
+      order: [
+        ['courseId', 'ASC'],
+        ['lessonNumber', 'ASC']
+      ]
+    });
+
+    const progressRows = await VocabCourseProgress.findAll({
+      where: { userId: user.id, courseId: { [Op.in]: courseIds } },
+      attributes: ['lessonId', 'completed']
+    });
+
+    const completedByLessonId = new Map();
+    for (const row of progressRows) {
+      const plain = row.get({ plain: true });
+      completedByLessonId.set(plain.lessonId, Boolean(plain.completed));
+    }
+
+    const lessonsByCourse = new Map();
+    for (const row of lessons) {
+      const plain = row.get({ plain: true });
+      const list = lessonsByCourse.get(plain.courseId) || [];
+      list.push(plain);
+      lessonsByCourse.set(plain.courseId, list);
+    }
+
+    const courses = [];
+    for (const meta of coursesMeta) {
+      const sorted = lessonsByCourse.get(meta.id) || [];
+      if (sorted.length === 0) {
+        courses.push({
+          courseId: meta.id,
+          title: meta.title,
+          currentLesson: null,
+          allLessonsCompleted: false
+        });
+        continue;
+      }
+
+      let current = null;
+      for (const lesson of sorted) {
+        if (!completedByLessonId.get(lesson.id)) {
+          current = lesson;
+          break;
+        }
+      }
+      if (!current) {
+        current = sorted[sorted.length - 1];
+      }
+
+      const allLessonsCompleted = sorted.every((lesson) => completedByLessonId.get(lesson.id) === true);
+
+      courses.push({
+        courseId: meta.id,
+        title: meta.title,
+        currentLesson: {
+          id: current.id,
+          lessonNumber: current.lessonNumber,
+          title: current.title || ''
+        },
+        allLessonsCompleted
+      });
+    }
+
+    return { courses };
+  }
+
   /**
    * Kurse, in die der Nutzer (per Hash) eingeschrieben ist — jede courseId nur einmal,
    * bei mehrfachen Einschreibungen zählt die jeweils neueste Zeile.