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/services/user_service.dart

@@ -367,7 +367,9 @@ class UserService {
     }
 
     final List<dynamic> data = jsonDecode(response.body);
-    return data.map((e) => ParentModel.fromJson(e)).toList();
+    return data
+        .map((e) => ParentModel.fromJson(Map<String, dynamic>.from(e as Map)))
+        .toList();
   }
 
   static Future<ParentModel> getParent(String userId) async {
@@ -379,7 +381,10 @@ class UserService {
       final err = jsonDecode(response.body) as Map<String, dynamic>?;
       throw Exception(_toStr(err?['message']) ?? 'Erreur chargement parent');
     }
-    return ParentModel.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
+    final decoded = jsonDecode(response.body);
+    return ParentModel.fromJson(
+      Map<String, dynamic>.from(decoded is Map ? decoded : {}),
+    );
   }
 
   static Future<ParentModel> updateParentFiche({
@@ -394,7 +399,7 @@ class UserService {
     if (response.statusCode != 200) {
       throw Exception(_extractErrorMessage(response.body, 'Erreur mise à jour parent'));
     }
-    return ParentModel.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
+    return _parentModelFromBody(response.body);
   }
 
   static Future<ParentModel> attachEnfantToParent({
@@ -410,7 +415,7 @@ class UserService {
     if (response.statusCode != 200 && response.statusCode != 201) {
       throw Exception(_extractErrorMessage(response.body, 'Erreur rattachement enfant'));
     }
-    return ParentModel.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
+    return _parentModelFromBody(response.body);
   }
 
   static Future<ParentModel> detachEnfantFromParent({
@@ -429,7 +434,7 @@ class UserService {
     if (response.body.isEmpty) {
       return getParent(parentUserId);
     }
-    return ParentModel.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
+    return _parentModelFromBody(response.body);
   }
 
   static Future<List<EnfantAdminModel>> getEnfants() async {
@@ -473,6 +478,13 @@ class UserService {
     return EnfantAdminModel.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
   }
 
+  static ParentModel _parentModelFromBody(String body) {
+    final decoded = jsonDecode(body);
+    return ParentModel.fromJson(
+      Map<String, dynamic>.from(decoded is Map ? decoded : {}),
+    );
+  }
+
   static String _extractErrorMessage(String body, String fallback) {
     try {
       final decoded = jsonDecode(body);