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/admin/admin-layout.tsx

@@ -24,6 +24,7 @@ import {
   UserCog,
   Bell,
 } from 'lucide-react';
+import { ThemeToggle } from '@/components/theme/theme-toggle';
 
 const menuItems = [
   {
@@ -264,6 +265,7 @@ export function AdminLayout({ children }: { children: React.ReactNode }) {
           <div className="flex-1" />
 
           <div className="flex items-center gap-4">
+            <ThemeToggle />
             <Link
               href="/"
               target="_blank"