Fix nonce logic in Customers and Content pages

[xecdev]

This PR fixes #63 by correcting the nonce logic for Customers and Content pages table sorting.

Test plan:

• Install and activate the updated plugin
• Go to Customers and Content pages → pages should render normally (no nonce check)
• Now in either Customers page or the Content page, sort the table by clicking a column from the table
• Once the page refereshes, the URL will now include the nonce, temper with it (or remove it from the URL) and you will see → “Security check failed”.
• As a non-admin, visit those admin pages using URL (with or without params) → no access.

[xecdev]

Ready for review