docs: update README intro and language switch

[wanghe-fit2cloud]

Summary

• Lead the project intro with the Jev-compatible positioning: "A self-hosted API and web interface for Laya's structured decision models, compatible with the TypeSafe Jev API format."
• Remove the "About LAYA SERVER" / "关于 LAYA SERVER" heading and place the intro directly under the title.
• Move the language switch to the top-left of both READMEs.

Notes

• Docs-only change; no code or configuration touched.