feat(94): focus() modal close button on open

[granttchart]

Description

Small a11y enhancement for #94 to focus() the modal close button when opened. In addition, ensures the close button is perfectly circular.

Checklist

• I am requesting to merge into the dev branch
• I have performed a self-review of my 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

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