feat(#131): en-tête fiche parent avec nom et co-parent.

Affiche le prénom/nom du parent en titre et le co-parent en sous-titre ; note handoff back dans archive/temporaires.

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

frontend/lib/models/parent_model.dart

@@ -3,11 +3,13 @@ import 'package:p_tits_pas/models/user.dart';
 
 class ParentModel {
   final AppUser user;
+  final AppUser? coParent;
   final int childrenCount;
   final List<ParentChildSummary> children;
 
   ParentModel({
     required this.user,
+    this.coParent,
     this.childrenCount = 0,
     this.children = const [],
   });
@@ -20,12 +22,19 @@ class ParentModel {
     }
     final user = AppUser.fromJson(userJson);
 
+    AppUser? coParent;
+    final coParentRaw = root['co_parent'];
+    if (coParentRaw is Map) {
+      coParent = AppUser.fromJson(Map<String, dynamic>.from(coParentRaw));
+    }
+
     final children = _parseChildren(root);
     final links = root['parentChildren'] ?? root['parent_children'];
     final linkCount = links is List ? links.length : 0;
 
     return ParentModel(
       user: user,
+      coParent: coParent,
       childrenCount: children.isNotEmpty ? children.length : linkCount,
       children: children,
     );