Feat/implement cache busting mechanism

[xecdev]

Implement a cache-busting mechanism for page reloads after a successful payment and login via PayButton. Instead of a standard reload, the URL is now appended with a unique timestamp query parameter (e.g., ?t=123456789) to force browsers and caching layers to fetch a fresh version of the page. This change has been applied in both assets/js/paywalled-content.js (after payment success) and assets/js/paybutton-paywall-cashtab-login.js (after login. This update addresses a rare caching issue reported by some users, where content would not appear unlocked until they performed a hard refresh.

To Test:

1. Payment Flow: Make a payment on your test site and verify that after success, the URL updates with a timestamp query parameter and the page reload shows unlocked content immediately.
2. Login Flow: Complete the login process and ensure the URL includes the cache-busting parameter. Confirm that the UI of the sticky header is changed and you see the Profile and Logout buttons.