feat(#140): liste enfants unifiée — cartes, âge précis et cadre scrollable

Partage AdminEnfantUserCard entre fiche parent et onglet Enfants, enrichit ParentChildSummary (photo, dates) et compacte la modale parent.

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

frontend/lib/utils/date_display_utils.dart

@@ -9,3 +9,71 @@ String formatIsoDateFr(String? s, {String ifEmpty = '–'}) {
     return s.trim();
   }
 }
+
+/// Parties d'âge calendaire entre [birth] et [reference] (date-only).
+({int years, int months, int days}) computeChildAgeParts(
+  DateTime birth,
+  DateTime reference,
+) {
+  final birthDay = DateTime(birth.year, birth.month, birth.day);
+  final refDay = DateTime(reference.year, reference.month, reference.day);
+
+  var years = refDay.year - birthDay.year;
+  var months = refDay.month - birthDay.month;
+  var days = refDay.day - birthDay.day;
+
+  if (days < 0) {
+    months--;
+    final prevMonth = DateTime(refDay.year, refDay.month, 0);
+    days += prevMonth.day;
+  }
+  if (months < 0) {
+    years--;
+    months += 12;
+  }
+
+  return (years: years, months: months, days: days);
+}
+
+String _yearLabel(int years) => years == 1 ? '1 an' : '$years ans';
+
+String _monthLabel(int months) => months == 1 ? '1 mois' : '$months mois';
+
+/// Libellé d'âge pour un enfant (liste admin, fiche parent, etc.).
+String formatChildAgeLabel({
+  String? birthDate,
+  String? dueDate,
+  String? status,
+}) {
+  if (status == 'a_naitre') {
+    final due = formatIsoDateFr(dueDate, ifEmpty: '');
+    if (due.isNotEmpty) return 'Naissance prévue : $due';
+    return 'À naître';
+  }
+
+  if (birthDate == null || birthDate.trim().isEmpty) return '';
+
+  try {
+    final birth = DateTime.parse(birthDate.trim().split('T').first);
+    final now = DateTime.now();
+    final parts = computeChildAgeParts(birth, now);
+
+    if (parts.years >= 1) {
+      if (parts.months > 0) {
+        return 'Âge : ${_yearLabel(parts.years)} ${parts.months} mois';
+      }
+      return 'Âge : ${_yearLabel(parts.years)}';
+    }
+    if (parts.months >= 1) {
+      return 'Âge : ${_monthLabel(parts.months)}';
+    }
+
+    final totalDays = DateTime(now.year, now.month, now.day)
+        .difference(DateTime(birth.year, birth.month, birth.day))
+        .inDays;
+    if (totalDays <= 0) return 'Âge : né récemment';
+    return 'Âge : $totalDays jour${totalDays > 1 ? 's' : ''}';
+  } catch (_) {
+    return 'Né le ${formatIsoDateFr(birthDate)}';
+  }
+}