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/scripts/create-bisaya-course-content.js

@@ -606,15 +606,10 @@ function buildContextGapExercise(lesson, didactics, pattern) {
   const scenarioPattern = didactics.corePatterns
     .map((entry) => corePatternTarget(entry))
     .find((candidate) => candidate && cue.includes(normalizeText(candidate).toLowerCase()));
+  // Do not hide a complete sentence behind one blank. Learners need enough
+  // visible context to know which word or short phrase belongs in the gap.
   const gapExercise = scenarioPattern
-    ? {
-        exerciseTypeId: 1,
-        title: `${lesson.title}: Kernmuster ergänzen`,
-        instruction: 'Fülle die Lücke mit dem passenden Bisaya-Ausdruck.',
-        questionData: { type: 'gap_fill', text: '{gap}', gaps: 1 },
-        answerData: { type: 'gap_fill', answers: [scenarioPattern] },
-        explanation: `Das Kernmuster lautet: "${scenarioPattern}".`
-      }
+    ? buildGapExercise(lesson.title, scenarioPattern)
     : buildGapExercise(lesson.title, pattern);
   if (!gapExercise) return null;