feat: Bandeau générique, dashboards et doc (squash develop, Closes #100)

- Bandeau générique (DashboardBandeau) pour Parent, Admin, Gestionnaire, AM
- ParentDashboardScreen, AdminDashboardScreen, GestionnaireDashboardScreen, AM dashboard
- AppFooter responsive, scripts Gitea (create/list issues parent API)
- Doc: ticket #101 Inscription Parent API, mise à jour 23_LISTE-TICKETS
- User.fromJson robustesse (nullable id/email/role)
- Suppression dashboard_app_bar.dart au profit de dashboard_bandeau.dart

Refs: #100, #101
Made-with: Cursor

frontend/lib/models/user.dart

@@ -41,9 +41,9 @@ class AppUser {
         relaisJson is Map<String, dynamic> ? relaisJson : <String, dynamic>{};
 
     return AppUser(
-      id: json['id'] as String,
-      email: json['email'] as String,
-      role: json['role'] as String,
+      id: (json['id'] as String?) ?? '',
+      email: (json['email'] as String?) ?? '',
+      role: (json['role'] as String?) ?? '',
       createdAt: json['cree_le'] != null
           ? DateTime.parse(json['cree_le'] as String)
           : (json['createdAt'] != null