feat: STEM Virtual Lab, English TTS companion, dynamic textbooks and zero-demo video fallback

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

@@ -201,6 +201,7 @@ class CurriculumLessonItemModel {
   final int durationSeconds;
   final int checkpointsCount;
   final bool isCompleted;
+  final bool hasVideo;
 
   const CurriculumLessonItemModel({
     required this.id,
@@ -210,6 +211,7 @@ class CurriculumLessonItemModel {
     this.durationSeconds = 1200, // 20 mins default
     this.checkpointsCount = 3,
     this.isCompleted = false,
+    this.hasVideo = false,
   });
 
   factory CurriculumLessonItemModel.fromJson(Map<String, dynamic> json) {
@@ -221,14 +223,21 @@ class CurriculumLessonItemModel {
       }
     }
 
+    final filePath = json['file']?.toString();
+    // Default video availability: true for seeded math lessons, false for unseeded physics/english
+    final bool hasVideoExplicit = (json['has_video'] as bool?) ?? 
+        ((json['video_url'] != null && json['video_url'].toString().isNotEmpty) ||
+        (filePath != null && filePath.contains('math_10/semester_1/unit_01/lesson_01')));
+
     return CurriculumLessonItemModel(
       id: json['id']?.toString() ?? 'lesson_${DateTime.now().millisecondsSinceEpoch}',
       title: json['title']?.toString() ?? 'درس بدون عنوان',
       outcomes: outs,
-      markdownFilePath: json['file']?.toString(),
+      markdownFilePath: filePath,
       durationSeconds: (json['duration_seconds'] as num?)?.toInt() ?? 1200,
       checkpointsCount: (json['checkpoints_count'] as num?)?.toInt() ?? 3,
       isCompleted: (json['is_completed'] as bool?) ?? false,
+      hasVideo: hasVideoExplicit,
     );
   }
 }