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

apps/student_app/lib/logic/cubits/video_playback_cubit.dart

@@ -1,4 +1,5 @@
 import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import '../../core/utils/app_logger.dart';
 import '../../data/models/socratic_checkpoint_model.dart';
 import '../../data/models/subject_model.dart';
@@ -81,11 +82,20 @@ class VideoPlaybackCubit extends Cubit<VideoPlaybackState> {
     emit(VideoPlaybackLoading());
     try {
       final playback = await _repo.getLessonPlayback(lesson.markdownFilePath ?? lesson.id, subjectId: subject?.id);
+      
+      // Load saved resume position
+      int resumePos = playback.lastPositionSeconds ?? 0;
+      try {
+        final prefs = await SharedPreferences.getInstance();
+        final localPos = prefs.getInt('saved_video_pos_${lesson.id}') ?? 0;
+        if (localPos > resumePos) resumePos = localPos;
+      } catch (_) {}
+
       emit(VideoPlaybackReady(
         playbackData: playback,
         lessonItem: lesson,
         subject: subject,
-        currentPositionSeconds: 0,
+        currentPositionSeconds: resumePos,
         isPlaying: true,
       ));
     } catch (e) {
@@ -155,13 +165,20 @@ class VideoPlaybackCubit extends Cubit<VideoPlaybackState> {
 
   Future<void> saveProgress({required int positionSeconds, required int watchedSeconds}) async {
     final currentState = state;
-    if (currentState is! VideoPlaybackReady || currentState.playbackData.lessonId <= 0) return;
+    if (currentState is! VideoPlaybackReady) return;
     try {
-      await _repo.saveProgress(
-        lessonId: currentState.playbackData.lessonId,
-        positionSeconds: positionSeconds,
-        watchedSeconds: watchedSeconds,
-      );
+      final prefs = await SharedPreferences.getInstance();
+      if (currentState.lessonItem != null) {
+        await prefs.setInt('saved_video_pos_${currentState.lessonItem!.id}', positionSeconds);
+      }
+      if (currentState.playbackData.lessonId > 0) {
+        await prefs.setInt('saved_video_pos_${currentState.playbackData.lessonId}', positionSeconds);
+        await _repo.saveProgress(
+          lessonId: currentState.playbackData.lessonId,
+          positionSeconds: positionSeconds,
+          watchedSeconds: watchedSeconds,
+        );
+      }
     } catch (e) {
       AppLogger.log('Progress sync deferred: $e', tag: 'VIDEO_CUBIT');
     }