feat(theme): add ThemeToggle and wire into header + admin top bar

- Add reusable client ThemeToggle at src/components/theme/theme-toggle.tsx
- Reads/writes novalon-theme localStorage key and toggles data-theme
- Placeholder on first render to avoid hydration mismatch
- Wired into public Header desktop cluster and AdminLayout top bar

src/components/layout/header.tsx

@@ -11,6 +11,7 @@ import { useSiteConfig, type NavigationItem } from '@/lib/site-config';
 import { MegaDropdown } from '@/components/layout/mega-dropdown';
 import { useFocusTrap } from '@/hooks/use-focus-trap';
 import { cn } from '@/lib/utils';
+import { ThemeToggle } from '@/components/theme/theme-toggle';
 
 function HeaderContent() {
   const [isOpen, setIsOpen] = useState(false);
@@ -153,6 +154,7 @@ function HeaderContent() {
             </nav>
 
             <div className="hidden md:flex items-center gap-3">
+              <ThemeToggle />
               <Button
                 size="sm"
                 asChild