feat: add admin users management page

- User list with search by name/email
- Change role (user/contributor/admin) via dropdown
- Ban/unban users with status indicators
- Uses better-auth admin plugin client methods

app/(admin)/admin/users/page.tsx

@@ -0,0 +1,10 @@
+import type { Metadata } from "next"
+import { UsersClient } from "./users-client"
+
+export const metadata: Metadata = {
+  title: "Users",
+}
+
+export default function UsersPage() {
+  return <UsersClient />
+}