Add accessibility tests, linting, and CI for floppydisk component

[cameronrye]

This PR improves the floppydisk UI library to be closer to production-ready:

• Phase 1: Accessibility updates

  • Keyboard activation via Enter/Space for the FloppyDisk component when not disabled
  • Focus-visible outline for keyboard users
  • Accessibility docs updated to describe behavior

• Phase 2: Testing and CI

  • Add Vitest + React Testing Library setup
  • Add FloppyDisk tests covering rendering, label behavior, aria-label override, and keyboard/mouse activation
  • Add minimal export smoke test for the built CJS bundle
  • GitHub Actions workflow: build, test, lint, and run the CJS export smoke test on pushes/PRs to main

• Phase 3: Linting/formatting and OSS polish

  • Add ESLint (TS + React) and Prettier with scripts: npm run lint and npm run format
  • Add LICENSE (MIT), CODE_OF_CONDUCT.md, and CONTRIBUTING.md
  • Update README to link to contributing and code of conduct docs and clarify license

All tests and lint pass locally via npm test and npm run lint.