Fix clics qui traversent les ecrans (z-index + background opaque)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/css/style.css

@@ -44,11 +44,14 @@ html, body {
   opacity: 0;
   pointer-events: none;
   transition: opacity 0.4s ease;
+  z-index: 1;
+  background: var(--fond);
 }
 
 .ecran.actif {
   opacity: 1;
   pointer-events: all;
+  z-index: 10;
 }
 
 /* === Ecran d'accueil === */

frontend/index.html

@@ -135,7 +135,7 @@
     </section>
 
     <!-- Choix admin : wizard ou parametres -->
-    <section id="ecran-admin-choix" class="ecran">
+    <section id="ecran-admin-choix" class="ecran" onclick="event.stopPropagation()">
       <h2>Administration</h2>
       <div class="grille-modes">
         <button class="btn-mode" onclick="ouvrirWizard()">