Fix student grade registration: set grade_10 as default, add auto-heal for legacy tawjihi_2008, add student grade selector and update-grade API

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

@@ -20,7 +20,9 @@ class ErrorNotebookSummary {
   });
 
   factory ErrorNotebookSummary.fromJson(Map<String, dynamic> json) {
-    final rawBySubject = json['by_subject'] as Map<String, dynamic>? ?? {};
+    final rawBySubject = json['by_subject'] is Map
+        ? Map<String, dynamic>.from(json['by_subject'] as Map)
+        : <String, dynamic>{};
     final bySub = <String, int>{};
     rawBySubject.forEach((k, v) {
       bySub[k] = (v is num) ? v.toInt() : 0;