fix: overlay and menu z-index, site-title back in header

- Restore #site-title inside #site-header
- Overlay below header (z-header - 2), menu below header (z-header - 1)
- Header stays above overlay; when menu-open, dim all header elements
  except #site-title (opacity trick)

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

assets/css/partials/_site-header.scss

@@ -1,21 +1,3 @@
-// Sorti du header pour pouvoir passer au-dessus de l'overlay
-#site-title {
-  position: fixed;
-  top: 0;
-  left: var(--padding-body);
-  height: var(--header-h);
-  width: 140px;
-  display: flex;
-  align-items: center;
-  z-index: calc(var(--z-header) + 3);
-
-  svg { width: 100px; }
-
-  @media #{$small} {
-    svg { width: 80px; }
-  }
-}
-
 #site-header {
 
 
@@ -53,7 +35,20 @@
     }
   }
 
+  #site-title {
+    position: relative;
+    top: -3px;
+    width: 140px;
 
+    svg{
+      width: 100px;
+    }
+
+    @media #{$small}{
+      svg{ width: 80px; }
+      top: -4px;
+    }
+  }
 
 
    #nav-highlight, 
@@ -288,4 +283,16 @@
 //       font-size: var(--fs-xsmall);
 //     }
 //   }
-// }
+// }
+
+// Quand le menu est ouvert : on dimme tout sauf le logo
+body.menu-open #site-header {
+  #nav-highlight,
+  #nav-investigation,
+  #theme-toggle,
+  #lang-toggle,
+  #menu-toggle {
+    opacity: 0.15;
+    transition: opacity .3s ease-in;
+  }
+}

assets/css/partials/_site-menu.scss

@@ -12,7 +12,7 @@
     background-color: var(--color-bg);
     border-left: var(--border-light);
     padding: var(--padding-body);
-    z-index: calc(var(--z-header) + 2);
+    z-index: calc(var(--z-header) - 1);
 
     @media #{$x-small}{ 
         width: 100vw; 
@@ -78,7 +78,7 @@
 #menu-overlay{
     position: fixed;
     inset: 0;
-    z-index: calc(var(--z-header) + 1);
+    z-index: calc(var(--z-header) - 2);
     cursor: pointer;
     background-color: rgba(0, 0, 0, 0.4);
     opacity: 0;