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/widgets/child_card_widget.dart

@@ -1,4 +1,5 @@
 import 'dart:math' as math;
+import 'dart:io';
 
 import 'package:flutter/material.dart';
 import 'package:google_fonts/google_fonts.dart';
@@ -20,7 +21,7 @@ bool _hasChildPhoto(ChildData c) {
   return registrationPhotoSlotHasImage(
     imageBytes: c.imageBytes,
     imageFile: c.imageFile,
-    imagePathOrAsset: null,
+    imagePathOrAsset: c.existingPhotoUrl,
   );
 }
 
@@ -33,6 +34,20 @@ Widget _buildChildPhotoImage(ChildData c, {required BoxFit fit}) {
   if (f != null) {
     return kIsWeb ? Image.network(f.path, fit: fit) : Image.file(f, fit: fit);
   }
+  final url = c.existingPhotoUrl?.trim();
+  if (url != null && url.isNotEmpty) {
+    if (url.startsWith('http://') || url.startsWith('https://')) {
+      return Image.network(url, fit: fit);
+    }
+    if (!kIsWeb) {
+      try {
+        final file = File(url);
+        if (file.existsSync()) {
+          return Image.file(file, fit: fit);
+        }
+      } catch (_) {}
+    }
+  }
   return Image.asset('assets/images/photo.png', fit: BoxFit.contain);
 }
 
@@ -221,6 +236,7 @@ class _ChildCardWidgetState extends State<ChildCardWidget> {
                 scaleFactor: scaleFactor,
                 imageBytes: widget.childData.imageBytes,
                 imageFile: widget.childData.imageFile,
+                imagePathOrAsset: widget.childData.existingPhotoUrl,
                 onTapPick: !config.isReadonly ? widget.onPickImage : null,
                 onClear: !config.isReadonly ? widget.onClearImage : null,
                 baseShadowColor: baseCardColorForShadow,

frontend/lib/widgets/registration_photo_slot.dart

@@ -71,6 +71,9 @@ class RegistrationPhotoSlot extends StatelessWidget {
       if (p.startsWith('assets/')) {
         return Image.asset(p, fit: fit);
       }
+      if (p.startsWith('http://') || p.startsWith('https://')) {
+        return Image.network(p, fit: fit);
+      }
       if (!kIsWeb) {
         try {
           final file = File(p);