feat(#140): fiches admin famille — IdentityBlock, parsing enfants et avatars web

Extrait IdentityBlock réutilisable, aligne les modales parent/enfant sur le style validation, corrige le parsing parentChildren et les URLs /uploads en Flutter web.

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

frontend/lib/models/enfant_admin_model.dart

@@ -89,8 +89,10 @@ class EnfantParentLink {
   EnfantParentLink({required this.parentId, this.parentName});
 
   factory EnfantParentLink.fromJson(Map<String, dynamic> json) {
-    final parent = json['parent'];
+    final parentId =
+        (json['parentId'] ?? json['id_parent'] ?? '').toString();
     String? name;
+    final parent = json['parent'];
     if (parent is Map<String, dynamic>) {
       final user = parent['user'];
       if (user is Map<String, dynamic>) {
@@ -99,7 +101,7 @@ class EnfantParentLink {
       }
     }
     return EnfantParentLink(
-      parentId: (json['parentId'] ?? json['id_parent'] ?? '').toString(),
+      parentId: parentId,
       parentName: name,
     );
   }