feat(admin): panneau Paramètres - sauvegarde config + test SMTP

- Onglet Paramètres dans l'admin avec 3 sections (Email, Personnalisation, Avancé)
- Service ConfigurationService (GET config, PATCH bulk, POST test-smtp)
- Bouton Sauvegarder et bouton Tester SMTP (sauvegarde avant test)
- Endpoints api_config pour configuration

Closes #15

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

frontend/lib/screens/administrateurs/admin_dashboardScreen.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:p_tits_pas/widgets/admin/assistante_maternelle_management_widget.dart';
 import 'package:p_tits_pas/widgets/admin/gestionnaire_management_widget.dart';
 import 'package:p_tits_pas/widgets/admin/parent_managmant_widget.dart';
+import 'package:p_tits_pas/widgets/admin/parametres_panel.dart';
 import 'package:p_tits_pas/widgets/app_footer.dart';
 import 'package:p_tits_pas/widgets/admin/dashboard_admin.dart';
 
@@ -9,15 +10,25 @@ class AdminDashboardScreen extends StatefulWidget {
   const AdminDashboardScreen({super.key});
 
   @override
-  _AdminDashboardScreenState createState() => _AdminDashboardScreenState();
+  State<AdminDashboardScreen> createState() => _AdminDashboardScreenState();
 }
 
 class _AdminDashboardScreenState extends State<AdminDashboardScreen> {
-  int selectedIndex = 0;
+  /// 0 = Gestion des utilisateurs, 1 = Paramètres
+  int mainTabIndex = 0;
 
-  void onTabChange(int index) {
+  /// Sous-onglet quand mainTabIndex == 0 : 0=Gestionnaires, 1=Parents, 2=AM, 3=Administrateurs
+  int subIndex = 0;
+
+  void onMainTabChange(int index) {
     setState(() {
-      selectedIndex = index;
+      mainTabIndex = index;
+    });
+  }
+
+  void onSubTabChange(int index) {
+    setState(() {
+      subIndex = index;
     });
   }
 
@@ -33,13 +44,18 @@ class _AdminDashboardScreenState extends State<AdminDashboardScreen> {
             ),
           ),
           child: DashboardAppBarAdmin(
-            selectedIndex: selectedIndex,
-            onTabChange: onTabChange,
+            selectedIndex: mainTabIndex,
+            onTabChange: onMainTabChange,
           ),
         ),
       ),
       body: Column(
         children: [
+          if (mainTabIndex == 0)
+            DashboardUserManagementSubBar(
+              selectedSubIndex: subIndex,
+              onSubTabChange: onSubTabChange,
+            ),
           Expanded(
             child: _getBody(),
           ),
@@ -50,17 +66,20 @@ class _AdminDashboardScreenState extends State<AdminDashboardScreen> {
   }
 
   Widget _getBody() {
-    switch (selectedIndex) {
+    if (mainTabIndex == 1) {
+      return const ParametresPanel();
+    }
+    switch (subIndex) {
       case 0:
-        return  const GestionnaireManagementWidget();
+        return const GestionnaireManagementWidget();
       case 1:
         return const ParentManagementWidget();
       case 2:
         return const AssistanteMaternelleManagementWidget();
       case 3:
-        return const Center(child: Text("👨‍💼 Administrateurs"));
+        return const Center(child: Text('👨‍💼 Administrateurs'));
       default:
-        return const Center(child: Text("Page non trouvée"));
+        return const Center(child: Text('Page non trouvée'));
     }
   }
 }