feat: add A2-B1 course expansion features including listening activities and conversation sessions

- Updated VocabCourseView.vue to display CEFR level for courses.
- Enhanced VocabLessonView.vue with new sections for listening activities and conversation labs, including audio controls and transcript toggling.
- Created migration to add new columns to vocab_course and new tables for conversation sessions and listening activities.
- Implemented models for vocab_conversation_session and vocab_listening_activity.
- Developed scripts to create new A2 and B1 bridge courses with structured lesson plans.
- Documented the A2-B1 expansion plan detailing course structure, learning goals, and assessment criteria.

backend/migrations-active/20260827000001-fix-bisaya-lesson-46-context-gap.cjs

@@ -0,0 +1,22 @@
+'use strict';
+
+/** Repairs the already generated exercise that rendered as a bare blank. */
+module.exports = {
+  async up(queryInterface) {
+    await queryInterface.sequelize.query(`
+      UPDATE community.vocab_grammar_exercise AS exercise
+      SET question_data = '{"type":"gap_fill","text":"{gap} ka padulong?","gaps":1}'::jsonb,
+          answer_data = '{"type":"gap_fill","answers":["Asa"]}'::jsonb,
+          explanation = 'Das Kernmuster lautet: "Asa ka padulong?"'
+      FROM community.vocab_course_lesson AS lesson
+      WHERE exercise.lesson_id = lesson.id
+        AND lesson.lesson_number = 46
+        AND lesson.title = 'Fragen im Alltag vertiefen'
+        AND exercise.title = 'Fragen im Alltag vertiefen: Kernmuster ergänzen'
+        AND exercise.question_data->>'text' = '{gap}';
+    `);
+  },
+  async down() {
+    // The old exercise has no useful learner-visible prompt and is not restored.
+  }
+};