Add basic accessibility (a11y)

[joelpeace48-cell]

Improve a11y: semantic HTML, aria-labels where needed, focus styles, and ensure keyboard navigation works for main flows.

[benedictfred]

@benedictfred has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

GM
Can I work on this issue? I will be done before the end of the day.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @benedictfred to this issue.