fix(nav): stop CTA press warp, pin user menu, hide PC hamburger

Lock top-bar button transforms on press; anchor avatar dropdown under
the trigger; show mobile drawer toggle only ≤767 and grid-layout the
mobile header so logo and menu no longer overlap.

assets/css/frontend/nav/zib-mobile-navbar.css

@@ -38,14 +38,16 @@
     margin-top: 5px;
   }
 
+  /* Center layout is grid-driven in nav.css (menu | logo | search).
+   Do NOT absolute-position brand — it overlaps the hamburger. */
   .navbar-top.center .navbar-brand {
-    position: absolute;
-    width: 100%;
+    position: static;
+    width: auto;
     text-align: center;
   }
 
   .navbar-top.center .navbar-brand + .navbar-toggle {
-    float: left;
+    float: none;
   }
 
   .navbar-top.center .navbar-brand > img {