[Modify-web] Add Web Accessiblity

Patch Note

Improved accessibility of the web xnb converter.

Changed buttons from div elements to button elements for better keyboard navigation and control using the Tab key.
Modified button colors to improve contrast, ensuring a 4.5:1 ratio between text and background for better readability by users with low vision.
Set empty alt attributes for image icons that already have accompanying captions.
Updated the alt text for preview images to describe the corresponding XNB file, enhancing screen reader accessibility.
Modified checkbox descriptions to appear on keyboard focus and added aria-describedby to ensure screen readers can read the descriptions.