feat: add admin layout with sidebar, auth guard, and navbar link

- Admin layout with server-side auth guard (redirects non-admins)
- Reusable sidebar component with active state detection
- Navbar profile dropdown shows admin link for admin users
- Admin routes are noindexed (robots: false)
- /admin redirects to /admin/users

components/admin/admin-sidebar.tsx

@@ -0,0 +1,40 @@
+"use client"
+
+import Link from "next/link"
+import { usePathname } from "next/navigation"
+import { UsersIcon, CpuIcon, Gamepad2Icon } from "lucide-react"
+
+const adminNavItems = [
+  { href: "/admin/users", label: "Users", icon: UsersIcon },
+  { href: "/admin/hardware", label: "Hardware", icon: CpuIcon },
+  { href: "/admin/games", label: "Games", icon: Gamepad2Icon },
+]
+
+export function AdminSidebar() {
+  const pathname = usePathname()
+
+  return (
+    <nav className="md:w-48 shrink-0">
+      <div className="flex md:flex-col gap-1 overflow-x-auto md:overflow-visible pb-2 md:pb-0 md:border-r md:border-border">
+        {adminNavItems.map((item) => {
+          const isActive =
+            pathname === item.href || pathname.startsWith(item.href + "/")
+          return (
+            <Link
+              key={item.href}
+              href={item.href}
+              className={`flex items-center gap-2 px-4 py-2.5 text-sm font-medium transition-colors whitespace-nowrap rounded-lg md:rounded-none md:border-l-2 md:border-r-0 md:border-transparent ${
+                isActive
+                  ? "bg-primary/10 text-primary md:border-l-primary"
+                  : "text-text/50 hover:text-text/70 hover:bg-text/5"
+              }`}
+            >
+              <item.icon className="h-4 w-4 shrink-0" />
+              {item.label}
+            </Link>
+          )
+        })}
+      </div>
+    </nav>
+  )
+}