Subjects per grade (math, calculus, geometry, stats, discrete), 5 new textbooks, 202 new islands, long islands split, hand-written figures and lesson texts for the new books

- content.Subject + Unit.subject, /api/home?subject=, tutor grounded in the chosen book
- 5 new books ingested; ~1150 hand-written questions, islands named by hand
- node_split: islands over 7 lessons split into parts of <=6 (lesson ids and progress kept)
- Lesson.figures hand-authored and visually checked for 653 lessons of the new books;
  gen_lesson_figures got --subject and survives network errors
- seed files re-exported

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

content/serializers.py

@@ -61,7 +61,24 @@ class StatsSerializer(serializers.Serializer):
     coins = serializers.IntegerField()
 
 
+class SubjectSerializer(serializers.Serializer):
+    key = serializers.CharField(help_text='همان که به `?subject=` داده می‌شود.')
+    title = serializers.CharField()
+    icon = serializers.CharField(allow_blank=True, help_text='نامِ آیکنِ lucide.')
+    color = serializers.CharField(allow_blank=True)
+    order = serializers.IntegerField()
+
+
 class HomeResponseSerializer(serializers.Serializer):
+    subject = SubjectSerializer(
+        allow_null=True,
+        help_text='کتابی که `units` از آن است؛ null فقط وقتی پایه هیچ فصلی ندارد.',
+    )
+    subjects = SubjectSerializer(
+        many=True,
+        help_text='همه‌ی کتاب‌های این پایه. فرانت نوارِ جابه‌جاییِ کتاب را فقط وقتی نشان می‌دهد '
+                  'که بیش از یکی باشد.',
+    )
     units = UnitWithNodesSerializer(many=True)
     stats = StatsSerializer()