Fix: form data not being sent in multi

[omar-ahmed42]

Description

Problem:

• When a form is submitted and multi downloads is expected, the file is sent successfully but not the form data/body as let postForm = document.querySelector('form[method="POST"]'); retrieves the first form found and it's mostly the CSRF form

Solution:

• Pass form to submitMultiPdfForm to use in FormData creation.

Closes #2490, #2488

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
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