fix(#112): afficher photos enfants en reprise + consentement cohérent

Charge existingPhotoUrl dans les cartes enfant (étapes 3 et 5) et pré-coche
le consentement photo lorsqu'une photo est déjà en base.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/models/user_registration_data.dart

@@ -30,6 +30,7 @@ class ParentData {
 class ChildData {
   static const Object _unsetImage = Object();
   static const Object _unsetImageBytes = Object();
+  static const Object _unsetExistingPhotoUrl = Object();
 
   String firstName;
   String lastName;
@@ -75,7 +76,7 @@ class ChildData {
     Object? imageBytes = _unsetImageBytes,
     CardColorVertical? cardColor,
     String? repriseChildId,
-    String? existingPhotoUrl,
+    Object? existingPhotoUrl = _unsetExistingPhotoUrl,
   }) {
     return ChildData(
       firstName: firstName ?? this.firstName,
@@ -90,7 +91,9 @@ class ChildData {
           identical(imageBytes, _unsetImageBytes) ? this.imageBytes : imageBytes as Uint8List?,
       cardColor: cardColor ?? this.cardColor,
       repriseChildId: repriseChildId ?? this.repriseChildId,
-      existingPhotoUrl: existingPhotoUrl ?? this.existingPhotoUrl,
+      existingPhotoUrl: identical(existingPhotoUrl, _unsetExistingPhotoUrl)
+          ? this.existingPhotoUrl
+          : existingPhotoUrl as String?,
     );
   }
 }