fix(better-auth): sign out code sample

[rbalet]

Which project does this relate to?

Start

Describe the bug

Starting an app with Biome & Better-auth, we get an error inside the better-auth/header-user.tsx file.

Your Example Website or App

NaN

Steps to Reproduce the Bug or Issue

1. Run npm create @tanstack/start@latest
2. Name your app
3. Select Biome
4. Select Nitro
5. Select Better-auth

Expected behavior

<button
  type="button"
  onClick={() => authClient.signOut()}
  className="flex-1 h-9 px-4 text-sm font-medium bg-white dark:bg-neutral-900 text-neutral-900 dark:text-neutral-50 border border-neutral-300 dark:border-neutral-700 hover:bg-neutral-50 dark:hover:bg-neutral-800 transition-colors"
>
  Sign out
</button>

Screenshots or Videos

Platform

• Router / Start Version: 1.132.0
• OS: macOS
• Bundler: vite
• Bundler Version: 7.1.7

Additional context

Can do a PR, just don't know where to find this code