Description

This Pull Request introduces unit tests for the shared visual Background component to ensure its rendering and accessibility properties remain correct and robust.

Specifically, the following changes were made:

• Modified the wrapper in Background.tsx to include aria-hidden="true", ensuring this purely decorative visual layer is hidden from assistive technologies.
• Created Background.test.tsx containing contract tests that verify the component renders without crashing, correctly applies custom state modifiers (active, error, idle), asserts the decorative layers are hidden, and remains independent of animation timing.

Related Issues

Closes #1414

Type of Change

• 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 not work as expected)
• Documentation update

How Has This Been Tested?

We validated these changes by running the unit tests using Vitest in the frontend directory:
```powershell
npx vitest run testing/unit/components/Background.test.tsx

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.