feat(#146–#149): sélection enfant/AM, capacité max et case libre.

Modales de rattachement partagées, filtre Libre/Sans garde, recalcul des places à l'attach, désactivation si capacité pleine, clic sur case libre.

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

frontend/lib/widgets/admin/common/admin_enfant_user_card.dart

@@ -34,6 +34,8 @@ class AdminEnfantUserCard extends StatelessWidget {
   final List<String> subtitleLines;
   final List<Widget> actions;
   final VoidCallback? onCardTap;
+  final EdgeInsetsGeometry? margin;
+  final EdgeInsetsGeometry? contentPadding;
 
   const AdminEnfantUserCard({
     super.key,
@@ -42,6 +44,8 @@ class AdminEnfantUserCard extends StatelessWidget {
     required this.subtitleLines,
     this.actions = const [],
     this.onCardTap,
+    this.margin,
+    this.contentPadding,
   });
 
   factory AdminEnfantUserCard.fromEnfant(
@@ -49,6 +53,8 @@ class AdminEnfantUserCard extends StatelessWidget {
     List<String> extraSubtitleLines = const [],
     List<Widget> actions = const [],
     VoidCallback? onCardTap,
+    EdgeInsetsGeometry? margin,
+    EdgeInsetsGeometry? contentPadding,
   }) {
     final parents = enfant.parentLinks
         .map((l) => l.parentName ?? 'Parent')
@@ -68,6 +74,8 @@ class AdminEnfantUserCard extends StatelessWidget {
       ),
       actions: actions,
       onCardTap: onCardTap,
+      margin: margin,
+      contentPadding: contentPadding,
     );
   }
 
@@ -76,6 +84,8 @@ class AdminEnfantUserCard extends StatelessWidget {
     List<String> extraSubtitleLines = const [],
     List<Widget> actions = const [],
     VoidCallback? onCardTap,
+    EdgeInsetsGeometry? margin,
+    EdgeInsetsGeometry? contentPadding,
   }) {
     return AdminEnfantUserCard(
       title: child.fullName,
@@ -88,6 +98,8 @@ class AdminEnfantUserCard extends StatelessWidget {
       ),
       actions: actions,
       onCardTap: onCardTap,
+      margin: margin,
+      contentPadding: contentPadding,
     );
   }
 
@@ -100,6 +112,8 @@ class AdminEnfantUserCard extends StatelessWidget {
       subtitleLines: subtitleLines,
       actions: actions,
       onCardTap: onCardTap,
+      margin: margin,
+      contentPadding: contentPadding,
     );
   }
 }