fix(admin): vignettes enfants du wizard validation (cover, sans fond blanc)

- BoxFit.cover pour remplir le cadre photo
- Suppression du fond blanc du slot, rayon factorisé

Made-with: Cursor

frontend/lib/widgets/admin/validation_family_wizard.dart

@@ -508,12 +508,12 @@ class _ValidationFamilyWizardState extends State<ValidationFamilyWizard> {
   }
 
   Widget _buildEnfantPhotoSlot(String photoUrl, double width, double height) {
+    const photoRadius = 8.0;
     return Container(
       width: width,
       height: height,
       decoration: BoxDecoration(
-        color: Colors.white,
-        borderRadius: BorderRadius.circular(8),
+        borderRadius: BorderRadius.circular(photoRadius),
         border: Border.all(color: Colors.black.withValues(alpha: 0.08)),
         boxShadow: [
           BoxShadow(
@@ -533,7 +533,7 @@ class _ValidationFamilyWizardState extends State<ValidationFamilyWizard> {
             )
           : AuthNetworkImage(
               url: photoUrl,
-              fit: BoxFit.contain,
+              fit: BoxFit.cover,
               width: width,
               height: height,
               errorBuilder: (_, __, ___) => ColoredBox(