feat: add English and Simplified Chinese i18n

[Rivbla]

Summary

• add centralized English and Simplified Chinese dictionaries
• auto-detect the browser language, with an explicit language switch stored in localStorage
• translate the HTML shell, runtime states, SVG labels, tooltips, errors, and theme controls
• include a minimal unprivileged Nginx image for self-hosted deployments
• preserve correct upload-test semantics by fully receiving the POST body before mapping the static-file 405 response to 200
• improve desktop and 390 × 844 mobile layout behavior

Validation

• npm test validates 51 bilingual keys and 50 runtime references
• JavaScript syntax, SVG XML, HTML, and git diff --check
• Nginx 1.31.3 configuration validation
• browser checks for automatic Chinese, manual switching, runtime labels, and desktop/mobile layouts

This is an unofficial community implementation and keeps the upstream MIT license intact.