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.
Desktop was rendering .mobile-header / .mobile-navbar in document flow
because Bootstrap 5 has no visible-xs-block and drawer CSS only ran
inside max-width:767px. Hide drawer shell by default; only show as fixed
off-canvas on small screens. Also fix orphan CSS rules in navbar-core,
give .navbar-search position:fixed so the overlay never reserves height.