fix: overlay above header, site-title above overlay

Move #site-title out of #site-header to its own stacking context,
enabling independent z-index control. Z-index order:
- #site-header: --z-header (2000)
- #menu-overlay: --z-header + 1
- #site-menu: --z-header + 2
- #site-title: --z-header + 3

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

assets/css/partials/_site-header.scss

@@ -1,3 +1,21 @@
+// 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 {
 
 
@@ -36,21 +54,6 @@
   }
 
 
-  #site-title {
-    position: relative;
-    top: -3px;
-
-    width: 140px;
-
-    svg{
-      width: 100px;
-    }
-
-    @media #{$small}{
-        svg{ width: 80px; }
-        top: -4px;
-      }
-  }
 
 
    #nav-highlight,