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/services/api/api_config.dart

@@ -14,6 +14,13 @@ class ApiConfig {
   static const String userProfile = '/users/profile';
   static const String userChildren = '/users/children';
 
+  // Configuration (admin)
+  static const String configuration = '/configuration';
+  static const String configurationSetupStatus = '/configuration/setup/status';
+  static const String configurationSetupComplete = '/configuration/setup/complete';
+  static const String configurationTestSmtp = '/configuration/test-smtp';
+  static const String configurationBulk = '/configuration/bulk';
+
   // Dashboard endpoints
   static const String dashboard = '/dashboard';
   static const String events = '/events';

frontend/lib/services/configuration_service.dart

@@ -0,0 +1,139 @@
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+import 'api/api_config.dart';
+import 'api/tokenService.dart';
+
+/// Réponse GET /configuration (liste complète)
+class ConfigItem {
+  final String cle;
+  final String? valeur;
+  final String type;
+  final String? categorie;
+  final String? description;
+
+  ConfigItem({
+    required this.cle,
+    this.valeur,
+    required this.type,
+    this.categorie,
+    this.description,
+  });
+
+  factory ConfigItem.fromJson(Map<String, dynamic> json) {
+    return ConfigItem(
+      cle: json['cle'] as String,
+      valeur: json['valeur'] as String?,
+      type: json['type'] as String? ?? 'string',
+      categorie: json['categorie'] as String?,
+      description: json['description'] as String?,
+    );
+  }
+}
+
+/// Réponse GET /configuration/:category (objet clé -> { value, type, description })
+class ConfigValueItem {
+  final dynamic value;
+  final String type;
+  final String? description;
+
+  ConfigValueItem({required this.value, required this.type, this.description});
+
+  factory ConfigValueItem.fromJson(Map<String, dynamic> json) {
+    return ConfigValueItem(
+      value: json['value'],
+      type: json['type'] as String? ?? 'string',
+      description: json['description'] as String?,
+    );
+  }
+}
+
+class ConfigurationService {
+  static Future<Map<String, String>> _headers() async {
+    final token = await TokenService.getToken();
+    return token != null
+        ? ApiConfig.authHeaders(token)
+        : Map<String, String>.from(ApiConfig.headers);
+  }
+
+  /// GET /api/v1/configuration/setup/status
+  static Future<bool> getSetupStatus() async {
+    final response = await http.get(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configurationSetupStatus}'),
+      headers: await _headers(),
+    );
+    if (response.statusCode != 200) return true;
+    final data = jsonDecode(response.body);
+    return data['data']?['setupCompleted'] as bool? ?? true;
+  }
+
+  /// GET /api/v1/configuration (toutes les configs)
+  static Future<List<ConfigItem>> getAll() async {
+    final response = await http.get(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configuration}'),
+      headers: await _headers(),
+    );
+    if (response.statusCode != 200) {
+      throw Exception(
+        (jsonDecode(response.body) as Map)['message'] ?? 'Erreur chargement configuration',
+      );
+    }
+    final data = jsonDecode(response.body);
+    final list = data['data'] as List<dynamic>? ?? [];
+    return list.map((e) => ConfigItem.fromJson(e as Map<String, dynamic>)).toList();
+  }
+
+  /// GET /api/v1/configuration/:category
+  static Future<Map<String, ConfigValueItem>> getByCategory(String category) async {
+    final response = await http.get(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configuration}/$category'),
+      headers: await _headers(),
+    );
+    if (response.statusCode != 200) {
+      throw Exception(
+        (jsonDecode(response.body) as Map)['message'] ?? 'Erreur chargement configuration',
+      );
+    }
+    final data = jsonDecode(response.body);
+    final map = data['data'] as Map<String, dynamic>? ?? {};
+    return map.map((k, v) => MapEntry(k, ConfigValueItem.fromJson(v as Map<String, dynamic>)));
+  }
+
+  /// PATCH /api/v1/configuration/bulk
+  static Future<void> updateBulk(Map<String, dynamic> body) async {
+    final response = await http.patch(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configurationBulk}'),
+      headers: await _headers(),
+      body: jsonEncode(body),
+    );
+    if (response.statusCode != 200) {
+      final err = jsonDecode(response.body) as Map;
+      throw Exception(err['message'] ?? 'Erreur lors de la sauvegarde');
+    }
+  }
+
+  /// POST /api/v1/configuration/test-smtp
+  static Future<String> testSmtp(String testEmail) async {
+    final response = await http.post(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configurationTestSmtp}'),
+      headers: await _headers(),
+      body: jsonEncode({'testEmail': testEmail}),
+    );
+    final data = jsonDecode(response.body) as Map;
+    if (response.statusCode == 200 && data['success'] == true) {
+      return data['message'] as String? ?? 'Test SMTP réussi.';
+    }
+    throw Exception(data['error'] ?? data['message'] ?? 'Échec du test SMTP');
+  }
+
+  /// POST /api/v1/configuration/setup/complete (après première config)
+  static Future<void> completeSetup() async {
+    final response = await http.post(
+      Uri.parse('${ApiConfig.baseUrl}${ApiConfig.configurationSetupComplete}'),
+      headers: await _headers(),
+    );
+    if (response.statusCode != 200) {
+      final err = jsonDecode(response.body) as Map;
+      throw Exception(err['message'] ?? 'Erreur finalisation configuration');
+    }
+  }
+}