Clear session cookie on logout for better security

[boomzero]

What does this PR aim to accomplish?:

This pull request introduces a small but important change to the XMOJ.user.js script. It adds functionality to clear a specific cookie (PHPSESSID) when a user logs out.

How does this PR accomplish the above?:

• XMOJ.user.js: Added a line to delete the PHPSESSID cookie by setting its expiration date to the past, ensuring the user's session is fully cleared during logout.

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[Copilot]

Pull Request Overview

This PR improves logout security by ensuring that the user's PHP session cookie is explicitly cleared during logout.

• It adds a line in XMOJ.user.js to expire the PHPSESSID cookie when the logout button is clicked.