fix(lightbox): make download button always visible and accessible for content type

[khushi19099]

Description

Made the 'Download' button in the Lightbox/Content view always visible and accessible for keyboard-only users. Updated the HTML to use a real <button> element with proper accessibility attributes, and added CSS to ensure the button is always visible and has a clear focus style.

Related Issue

Fixes the accessibility issue where the 'Download' button was only visible on hover and not accessible to keyboard users.

Motivation and Context

This change is required to meet accessibility standards, ensuring that all users, including those using keyboards or assistive technologies, can discover and use the 'Download' button in the Lightbox/Content view.

How Has This Been Tested?

• Manually tested by tabbing through the Lightbox/Content view to ensure the 'Download' button is always visible and focusable.
• Verified that the button is announced correctly by screen readers.
• Checked that the button is operable using keyboard navigation.

Screenshots:

Before (If applicable):

Button only visible on mouse hover.

After:

Button is always visible and has a visible focus outline when tabbed to.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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