feat: redesign admin sidebar with grouped sections

- Add Management and Moderation sections with labeled groups
- Section labels: muted uppercase micro text
- Section dividers: horizontal on desktop, vertical on mobile
- Active state: bg-primary/10 text-primary rounded-lg
- Widen desktop sidebar from md:w-48 to md:w-56
- Add desktop-only Admin Panel header with LayoutDashboardIcon
- Expand nav to cover all 6 admin pages (users, hardware, games,
  reports, benchmarks, comments)
- Update admin layout description to 'Manage your platform'

app/(admin)/layout.tsx

@@ -20,7 +20,7 @@ export default async function AdminLayout({
                 <div className='max-w-7xl mx-auto'>
                     <h1 className='text-2xl sm:text-3xl font-bold'>Admin</h1>
                     <p className='text-sm text-text/60 mt-1'>
-                        Manage users, hardware, and games
+                        Manage your platform
                     </p>
                 </div>
             </div>