fix: Button size and layout in sandbox

[remi-guan]

Description

Changed the button width and centered the dropdown arrow, see below images.

Related Issue

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
• Code refactoring
• Performance improvement

How Has This Been Tested?

Screenshots (if applicable)

Before

After

Checklist

• My code follows the project's style guidelines
• 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.