Localize demo and make Chinese the default

[VeteranBoLuo]

What changed

• make Simplified Chinese the default README and restore English as README.en.md
• add separate live-demo and raw-API entry points to every README
• add a dated mainland-China compatibility note based on 12 Chinese and 36 international sites
• localize the interactive demo in Simplified Chinese, English, Japanese, and Korean
• select the browser language when supported, default unknown languages to Chinese, and remember manual choices
• use a mixed set of Chinese and international example domains
• point the GitHub repository homepage to the interactive demo

Why

The project is primarily intended for users in China, while still supporting international developers and websites. The previous README button opened a raw image response instead of explaining or demonstrating the product, and the demo itself had no language controls.

Validation

• npm run check — 13 tests passed
• embedded demo script compilation test
• simulated Chinese default and English language switch
• local HTTP smoke test — demo 200, prefixed health 200, Baidu favicon 200
• README language-link and stale-reference checks