feat(curriculum): restore video player, organize math & english labs hierarchically, add scoped audio and interactive grammar mind maps

apps/student_app/lib/data/models/socratic_checkpoint_model.dart

@@ -139,6 +139,28 @@ class LessonPlaybackData {
       lastPositionSeconds: lastPos?.toInt(),
     );
   }
+
+  LessonPlaybackData copyWith({
+    int? lessonId,
+    String? title,
+    int? durationSeconds,
+    String? videoUrl,
+    String? storageType,
+    List<LessonVersionModel>? availableVersions,
+    List<SocraticCheckpointModel>? checkpoints,
+    int? lastPositionSeconds,
+  }) {
+    return LessonPlaybackData(
+      lessonId: lessonId ?? this.lessonId,
+      title: title ?? this.title,
+      durationSeconds: durationSeconds ?? this.durationSeconds,
+      videoUrl: videoUrl ?? this.videoUrl,
+      storageType: storageType ?? this.storageType,
+      availableVersions: availableVersions ?? this.availableVersions,
+      checkpoints: checkpoints ?? this.checkpoints,
+      lastPositionSeconds: lastPositionSeconds ?? this.lastPositionSeconds,
+    );
+  }
 }
 
 /// Model for Teacher vs AI Lesson versions