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/widgets/admin/dashboard_admin.dart

@@ -1,47 +1,47 @@
 import 'package:flutter/material.dart';
 
+/// Barre principale du dashboard admin : 2 onglets (Gestion des utilisateurs | Paramètres) + infos utilisateur.
 class DashboardAppBarAdmin extends StatelessWidget implements PreferredSizeWidget {
   final int selectedIndex;
   final ValueChanged<int> onTabChange;
 
-  const DashboardAppBarAdmin({Key? key, required this.selectedIndex, required this.onTabChange}) : super(key: key);
+  const DashboardAppBarAdmin({
+    Key? key,
+    required this.selectedIndex,
+    required this.onTabChange,
+  }) : super(key: key);
 
   @override
   Size get preferredSize => const Size.fromHeight(kToolbarHeight + 10);
 
   @override
   Widget build(BuildContext context) {
-    final isMobile = MediaQuery.of(context).size.width < 768;
     return AppBar(
       elevation: 0,
       automaticallyImplyLeading: false,
       title: Row(
         children: [
-          SizedBox(width: MediaQuery.of(context).size.width * 0.19),
-          const Text(
-            "P'tit Pas",
-            style: TextStyle(
-              color: Color(0xFF9CC5C0),
-              fontWeight: FontWeight.bold,
-              fontSize: 18,
+          const SizedBox(width: 24),
+          Image.asset(
+            'assets/images/logo.png',
+            height: 40,
+            fit: BoxFit.contain,
+          ),
+          Expanded(
+            child: Center(
+              child: Row(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  _buildNavItem(context, 'Gestion des utilisateurs', 0),
+                  const SizedBox(width: 24),
+                  _buildNavItem(context, 'Paramètres', 1),
+                ],
+              ),
             ),
           ),
-          SizedBox(width: MediaQuery.of(context).size.width * 0.1),
-
-          // Navigation principale
-          _buildNavItem(context, 'Gestionnaires', 0),
-          const SizedBox(width: 24),
-          _buildNavItem(context, 'Parents', 1),
-          const SizedBox(width: 24),
-          _buildNavItem(context, 'Assistantes maternelles', 2),
-          const SizedBox(width: 24),
-          _buildNavItem(context, 'Administrateurs', 3),
         ],
       ),
-      actions: isMobile
-          ? [_buildMobileMenu(context)]
-          : [
-        // Nom de l'utilisateur
+      actions: [
         const Padding(
           padding: EdgeInsets.symmetric(horizontal: 16),
           child: Center(
@@ -55,8 +55,6 @@ class DashboardAppBarAdmin extends StatelessWidget implements PreferredSizeWidge
             ),
           ),
         ),
-        
-        // Bouton déconnexion
         Padding(
           padding: const EdgeInsets.only(right: 16),
           child: TextButton(
@@ -72,51 +70,30 @@ class DashboardAppBarAdmin extends StatelessWidget implements PreferredSizeWidge
             child: const Text('Se déconnecter'),
           ),
         ),
-        SizedBox(width: MediaQuery.of(context).size.width * 0.1),
       ],
     );
   }
 
   Widget _buildNavItem(BuildContext context, String title, int index) {
-  final bool isActive = index == selectedIndex;
-  return InkWell(
-    onTap: () => onTabChange(index),
-    child: Container(
-      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
-      decoration: BoxDecoration(
-        color: isActive ? const Color(0xFF9CC5C0) : Colors.transparent,
-        borderRadius: BorderRadius.circular(20),
-        border: isActive ? null : Border.all(color: Colors.black26),
-      ),
-      child: Text(
-        title,
-        style: TextStyle(
-          color: isActive ? Colors.white : Colors.black,
-          fontWeight: isActive ? FontWeight.w600 : FontWeight.normal,
-          fontSize: 14,
+    final bool isActive = index == selectedIndex;
+    return InkWell(
+      onTap: () => onTabChange(index),
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+        decoration: BoxDecoration(
+          color: isActive ? const Color(0xFF9CC5C0) : Colors.transparent,
+          borderRadius: BorderRadius.circular(20),
+          border: isActive ? null : Border.all(color: Colors.black26),
+        ),
+        child: Text(
+          title,
+          style: TextStyle(
+            color: isActive ? Colors.white : Colors.black,
+            fontWeight: isActive ? FontWeight.w600 : FontWeight.normal,
+            fontSize: 14,
+          ),
         ),
       ),
-    ),
-  );
-}
-
-
-  Widget _buildMobileMenu(BuildContext context) {
-    return PopupMenuButton<int>(
-      icon: const Icon(Icons.menu, color: Colors.white),
-      onSelected: (value) {
-        if (value == 4) {
-          _handleLogout(context);
-        }
-      },
-      itemBuilder: (context) => [
-        const PopupMenuItem(value: 0, child: Text("Gestionnaires")),
-        const PopupMenuItem(value: 1, child: Text("Parents")),
-        const PopupMenuItem(value: 2, child: Text("Assistantes maternelles")),
-        const PopupMenuItem(value: 3, child: Text("Administrateurs")),
-        const PopupMenuDivider(),
-        const PopupMenuItem(value: 4, child: Text("Se déconnecter")),
-      ],
     );
   }
 
@@ -142,4 +119,65 @@ class DashboardAppBarAdmin extends StatelessWidget implements PreferredSizeWidge
       ),
     );
   }
-}
+}
+
+/// Sous-barre affichée quand "Gestion des utilisateurs" est actif : 4 onglets sans infos utilisateur.
+class DashboardUserManagementSubBar extends StatelessWidget {
+  final int selectedSubIndex;
+  final ValueChanged<int> onSubTabChange;
+
+  const DashboardUserManagementSubBar({
+    Key? key,
+    required this.selectedSubIndex,
+    required this.onSubTabChange,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      height: 48,
+      decoration: BoxDecoration(
+        color: Colors.grey.shade100,
+        border: Border(bottom: BorderSide(color: Colors.grey.shade300)),
+      ),
+      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 6),
+      child: Center(
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            _buildSubNavItem(context, 'Gestionnaires', 0),
+            const SizedBox(width: 16),
+            _buildSubNavItem(context, 'Parents', 1),
+            const SizedBox(width: 16),
+            _buildSubNavItem(context, 'Assistantes maternelles', 2),
+            const SizedBox(width: 16),
+            _buildSubNavItem(context, 'Administrateurs', 3),
+          ],
+        ),
+      ),
+    );
+  }
+
+  Widget _buildSubNavItem(BuildContext context, String title, int index) {
+    final bool isActive = index == selectedSubIndex;
+    return InkWell(
+      onTap: () => onSubTabChange(index),
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 14, vertical: 6),
+        decoration: BoxDecoration(
+          color: isActive ? const Color(0xFF9CC5C0) : Colors.transparent,
+          borderRadius: BorderRadius.circular(16),
+          border: isActive ? null : Border.all(color: Colors.black26),
+        ),
+        child: Text(
+          title,
+          style: TextStyle(
+            color: isActive ? Colors.white : Colors.black87,
+            fontWeight: isActive ? FontWeight.w600 : FontWeight.normal,
+            fontSize: 13,
+          ),
+        ),
+      ),
+    );
+  }
+}