feat(vocab): enhance lesson state management and persistence in VocabLessonView

- Added a new JSONB field `lessonState` to the VocabCourseProgress model to store detailed lesson state information.
- Implemented methods in VocabService for sanitizing and serializing lesson state, ensuring robust data handling.
- Updated VocabLessonView to manage lesson state persistence, including local storage and server synchronization, improving user experience during vocabulary lessons.
- Introduced mechanisms for exporting and normalizing exercise answers, enhancing the accuracy of saved progress.

backend/migrations/20260401000000-add-vocab-course-progress-lesson-state.cjs

@@ -0,0 +1,20 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface) {
+    await queryInterface.sequelize.query(`
+      ALTER TABLE community.vocab_course_progress
+      ADD COLUMN IF NOT EXISTS lesson_state JSONB NOT NULL DEFAULT '{}'::jsonb;
+
+      COMMENT ON COLUMN community.vocab_course_progress.lesson_state IS
+        'Persistierter UI- und Lernzustand pro Nutzer und Lektion fuer Resume im Sprachkurs.';
+    `);
+  },
+
+  async down(queryInterface) {
+    await queryInterface.sequelize.query(`
+      ALTER TABLE community.vocab_course_progress
+      DROP COLUMN IF EXISTS lesson_state;
+    `);
+  }
+};