merge head/nav templates into unified head.php + header.php; semantic CSS for nav

public/admin/index.php

@@ -30,7 +30,8 @@ try {
     die("Erreur lors du chargement de la liste.");
 }
 ?>
-<?php require_once APP_ROOT . '/templates/admin/head.php'; ?>
+<?php $isAdmin = true; $bodyClass = 'admin-body'; require_once APP_ROOT . '/templates/head.php'; ?>
+    <?php include APP_ROOT . '/templates/header.php'; ?>
 
 <script>
 function toggleAll(src) {