Potential fix for code scanning alert no. 11: DOM text reinterpreted as HTML (#3497)

Potential fix for
[https://github.com/Stirling-Tools/Stirling-PDF/security/code-scanning/11](https://github.com/Stirling-Tools/Stirling-PDF/security/code-scanning/11)

To fix the issue, we should avoid using `innerHTML` to insert untrusted
data into the DOM. Instead, we can use DOM manipulation methods like
`createElement` and `appendChild` to construct the required HTML
structure safely. These methods do not interpret strings as HTML,
thereby mitigating the risk of XSS.

Specifically:
1. Replace the `innerHTML` assignment on line 302 with code that creates
the required DOM elements programmatically.
2. Ensure that the `selectedOperation` value is inserted as plain text
using `textContent` or equivalent methods.

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