a11y: label api key create form

[es3298]

Summary

• replace the API key label input's placeholder/aria-label-only treatment with the shared TextField component
• group the create controls in a labelled fieldset so assistive tech exposes the form purpose
• reuse the shared Button styling and document the accessibility behavior
• add focused tests for the labelled group, associated field label, and successful create flow/status announcement

Closes #123

Validation

• npm test -- src/app/api-keys/page.test.tsx --runInBand
• npm run lint
• git diff --check

Notes

• npm run build is still blocked by the existing upstream src/app/quote/Client.tsx assetsDiffer type error.
• npm test -- --runInBand is still blocked by existing upstream assetsDiffer quote test failures and the existing theme storage no-op expectation failure.