feat(inscription): email accusé réception, photos enfants, formulaires identité

- Backend: mail après inscription parent avec n° dossier, UPLOAD_PHOTOS_DIR, réponse API
- Frontend: imageBytes + payload photo, utils email/code postal/téléphone, champs dédiés
- Formulaires admin, login (focus), personal_info, child_card, custom_app_text_field

Made-with: Cursor

frontend/lib/widgets/admin/relais_management_panel.dart

@@ -482,6 +482,9 @@ class _RelaisFormDialogState extends State<_RelaisFormDialog> {
     if (!_isValidPostalCode(_postalCodeCtrl.text.trim())) {
       return false;
     }
+    if (validateFrenchNationalPhone(_ligneFixeCtrl.text, allowEmpty: true) != null) {
+      return false;
+    }
 
     for (final day in _days) {
       final ferme = _closedByDay[day] ?? false;
@@ -721,11 +724,7 @@ class _RelaisFormDialogState extends State<_RelaisFormDialog> {
         TextField(
           controller: _ligneFixeCtrl,
           keyboardType: TextInputType.phone,
-          inputFormatters: [
-            FilteringTextInputFormatter.digitsOnly,
-            LengthLimitingTextInputFormatter(10),
-            FrenchPhoneNumberFormatter(),
-          ],
+          inputFormatters: frenchPhoneInputFormatters,
           decoration: const InputDecoration(
             labelText: 'Ligne fixe',
             hintText: '01 23 45 67 89',