feat: click anywhere on header closes menu

- Header click removes menu-open (stopPropagation on toggle button
  to preserve its toggle behavior)
- cursor: pointer on site-header when menu is open

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/partials/_site-header.scss

@@ -248,6 +248,8 @@
 
 // Quand le menu est ouvert : on dimme tout sauf le logo
 body.menu-open #site-header {
+  cursor: pointer;
+
   #nav-highlight,
   #nav-investigation,
   #theme-toggle,