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

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

@@ -85,6 +85,7 @@ class LessonPlaybackData {
   final String storageType;
   final List<LessonVersionModel> availableVersions;
   final List<SocraticCheckpointModel> checkpoints;
+  final int? lastPositionSeconds;
 
   const LessonPlaybackData({
     required this.lessonId,
@@ -94,6 +95,7 @@ class LessonPlaybackData {
     this.storageType = 'api_upload',
     this.availableVersions = const [],
     this.checkpoints = const [],
+    this.lastPositionSeconds,
   });
 
   factory LessonPlaybackData.fromJson(Map<String, dynamic> json) {
@@ -124,6 +126,8 @@ class LessonPlaybackData {
       playback['hls_url']?.toString() ?? playback['stream_url']?.toString() ?? playback['video_url']?.toString() ?? '',
     );
 
+    final lastPos = (json['last_position_seconds'] ?? playback['last_position_seconds'] ?? json['progress']?['position_seconds']) as num?;
+
     return LessonPlaybackData(
       lessonId: (lesson['id'] as num?)?.toInt() ?? 0,
       title: lesson['title']?.toString() ?? 'الدرس التفاعلي',
@@ -132,6 +136,7 @@ class LessonPlaybackData {
       storageType: playback['storage_type']?.toString() ?? 'api_upload',
       availableVersions: versions,
       checkpoints: points,
+      lastPositionSeconds: lastPos?.toInt(),
     );
   }
 }