feat(#112): aligner reprise front sur dossier complet GET/PATCH

Préremplit parents, enfants, motivation et fiche AM depuis reprise-dossier
et envoie le body PATCH complet (co-parent, enfants par id, champs pro AM).

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

frontend/lib/screens/auth/am_register_step4_screen.dart

@@ -9,6 +9,7 @@ import '../../models/card_assets.dart';
 import '../../config/display_config.dart';
 import '../../services/auth_service.dart';
 import '../../services/reprise_session.dart';
+import '../../utils/reprise_payload.dart';
 import '../../widgets/hover_relief_widget.dart';
 import '../../widgets/image_button.dart';
 import '../../widgets/custom_navigation_button.dart';
@@ -59,14 +60,11 @@ class _AmRegisterStep4ScreenState extends State<AmRegisterStep4Screen> {
     try {
       if (RepriseSession.isActive) {
         await AuthService.resoumettreReprise(
-          token: RepriseSession.token!,
-          prenom: registrationData.firstName,
-          nom: registrationData.lastName,
-          telephone: registrationData.phone,
-          adresse: registrationData.streetAddress,
-          ville: registrationData.city,
-          codePostal: registrationData.postalCode,
-          photoUrl: RepriseSession.photoUrl,
+          await ReprisePayload.amPatch(
+            registrationData,
+            RepriseSession.token!,
+            existingPhotoUrl: RepriseSession.photoUrlForApi,
+          ),
         );
         RepriseSession.clear();
         if (!mounted) return;