fix(cli): reject cross-site loopback writes

[xingkaixin]

Summary

• guard every unauthenticated loopback API mutation before route handlers run
• require JSON media types for POST, PUT, and PATCH requests
• reject mismatched Origin values and Fetch Metadata sites other than same-origin or none
• preserve bodyless DELETE requests and non-browser clients that omit browser-only metadata

Security impact

This blocks cross-site pages from importing bookmarks or injecting client logs through CORS-safelisted POST bodies. The Fetch Metadata policy follows the W3C Sec-Fetch-Site relationship model: https://www.w3.org/TR/fetch-metadata/

Test plan

• pnpm lint
• pnpm format:check
• pnpm test
• pnpm build

Issue: CS-193