Build a unit converter as a static web app in this directory: index.html, style.css, convert.js. Categories: length, mass, temperature, data size, time. Instant two-way conversion between any pair in a category, swap button, precision selector, recent-conversions list in localStorage, keyboard-first UX, dark theme, and tests.html + tests.js exercising the conversion math including temperature edge cases. Run the tests headless with node and fix what fails.