[Nigh]

Summary

• add global Simplified Chinese and English localization across the UI, metadata, errors, help, and autocomplete
• replace the binary language toggle with an extensible, accessible dropdown with an opaque themed background
• make the English README the default and add a linked Simplified Chinese README
• update end-to-end coverage and project context documentation

Verification

• npm test -- --run (70 tests passed)
• npm run check (0 errors)
• targeted Playwright test could not run locally because the configured Chromium executable is not installed; CI will run the browser suite