feat: Update child photo label from Photo URL to Child Image

Changed translation key from "Photo URL (Optional)" to "Child Image (Optional)"
in all supported languages:
- English: Child Image (Optional)
- Spanish: Imagen del Niño (Opcional)
- French: Image de l'Enfant (Optionnel)
- Portuguese: Imagem da Criança (Opcional)
- German: Kinderbild (Optional)
- Italian: Immagine Bambino (Opzionale)
- Chinese: 儿童照片（可选）

This makes it clearer that users should upload a child's photo, not a URL.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-web/locales/zh/children.json

@@ -17,7 +17,7 @@
     "name": "姓名",
     "birthDate": "出生日期",
     "gender": "性别",
-    "photoUrl": "照片URL（可选）",
+    "photoUrl": "儿童照片（可选）",
     "photoPlaceholder": "https://example.com/photo.jpg",
     "cancel": "取消",
     "add": "添加",