feat(#132): création enfant staff (onglet Enfants) — front + back.

Squash depuis develop : POST /enfants staff + parent_user_id, photo
multipart, modale création + sélection famille, fixes UX/birth_date.

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

frontend/lib/utils/date_display_utils.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/services.dart';
 import 'package:intl/intl.dart';
 import 'package:p_tits_pas/utils/enfant_status_utils.dart';
 
@@ -11,10 +12,50 @@ String formatIsoDateFr(String? s, {String ifEmpty = '–'}) {
   }
 }
 
-/// Convertit `dd/MM/yyyy` (ou ISO) en `yyyy-MM-dd` pour l'API.
+/// Affiche une date ISO en saisie guidée `jj / mm / aaaa`.
+String formatIsoDateFrInput(String? s, {String ifEmpty = ''}) {
+  if (s == null || s.trim().isEmpty) return ifEmpty;
+  try {
+    final dt = DateTime.parse(s.trim());
+    return formatFrenchDateDigits(
+      DateFormat('ddMMyyyy').format(dt),
+    );
+  } catch (_) {
+    return formatFrenchDateDigits(s.replaceAll(RegExp(r'\D'), ''));
+  }
+}
+
+/// Formate jusqu’à 8 chiffres en `jj / mm / aaaa`.
+String formatFrenchDateDigits(String digits) {
+  final d = digits.replaceAll(RegExp(r'\D'), '');
+  final limited = d.length > 8 ? d.substring(0, 8) : d;
+  final buf = StringBuffer();
+  for (var i = 0; i < limited.length; i++) {
+    if (i == 2 || i == 4) buf.write(' / ');
+    buf.write(limited[i]);
+  }
+  return buf.toString();
+}
+
+/// Convertit `dd/MM/yyyy`, `dd / MM / yyyy`, 8 chiffres ou ISO en `yyyy-MM-dd`.
 String? parseFrDateToIso(String text) {
   final t = text.trim();
   if (t.isEmpty) return null;
+
+  final digits = t.replaceAll(RegExp(r'\D'), '');
+  if (digits.length == 8) {
+    final dd = digits.substring(0, 2);
+    final mm = digits.substring(2, 4);
+    final yyyy = digits.substring(4, 8);
+    try {
+      return DateFormat('dd/MM/yyyy')
+          .parseStrict('$dd/$mm/$yyyy')
+          .toIso8601String()
+          .split('T')
+          .first;
+    } catch (_) {}
+  }
+
   try {
     return DateFormat('dd/MM/yyyy')
         .parseStrict(t)
@@ -96,3 +137,43 @@ String formatChildAgeLabel({
     return 'Né le ${formatIsoDateFr(birthDate)}';
   }
 }
+
+/// Saisie date FR : 8 chiffres → `jj / mm / aaaa`.
+class FrenchDateInputFormatter extends TextInputFormatter {
+  const FrenchDateInputFormatter();
+
+  @override
+  TextEditingValue formatEditUpdate(
+    TextEditingValue oldValue,
+    TextEditingValue newValue,
+  ) {
+    final digits = newValue.text.replaceAll(RegExp(r'\D'), '');
+    final limited = digits.length > 8 ? digits.substring(0, 8) : digits;
+    final formatted = formatFrenchDateDigits(limited);
+
+    final digitsBeforeCursor = newValue.text
+        .substring(0, newValue.selection.start.clamp(0, newValue.text.length))
+        .replaceAll(RegExp(r'\D'), '')
+        .length
+        .clamp(0, limited.length);
+
+    return TextEditingValue(
+      text: formatted,
+      selection: TextSelection.collapsed(
+        offset: _cursorOffset(formatted, digitsBeforeCursor),
+      ),
+    );
+  }
+
+  static int _cursorOffset(String formatted, int digitsBeforeCursor) {
+    if (digitsBeforeCursor <= 0) return 0;
+    var seen = 0;
+    for (var i = 0; i < formatted.length; i++) {
+      if (RegExp(r'\d').hasMatch(formatted[i])) {
+        seen++;
+        if (seen >= digitsBeforeCursor) return i + 1;
+      }
+    }
+    return formatted.length;
+  }
+}