fix: admin panel template literal — ' + t() + ' → ${t()}

The concatenation syntax was literal text inside backtick template,
showing raw code on screen. Fixed both admin.title and admin.new_user.

frontend/js/auth.js

@@ -354,7 +354,7 @@ const AdminPanel = {
     this._modal.innerHTML = `
       <div class="editor-container">
         <div class="editor-header">
-          <div class="editor-title">⚙️ ' + t('admin.title') + '</div>
+          <div class="editor-title">⚙️ ${t('admin.title')}</div>
           <div class="editor-actions">
             <button class="editor-btn" id="admin-close" title="Fermer">
               <i data-lucide="x" style="width:16px;height:16px"></i>
@@ -363,7 +363,7 @@ const AdminPanel = {
         </div>
         <div class="editor-body" id="admin-body">
           <div class="admin-toolbar">
-            <button class="btn-login" id="admin-add-user" style="font-size:0.85rem;padding:6px 16px;">' + t('admin.new_user') + '</button>
+            <button class="btn-login" id="admin-add-user" style="font-size:0.85rem;padding:6px 16px;">${t('admin.new_user')}</button>
           </div>
           <div id="admin-users-list" class="admin-users-list"></div>
         </div>