[Feature] Add SEO metadata and polish MeteorTest page

[ConnorQi01]

Summary

Add localized SEO metadata and reshape the MeteorTest page into a clearer product-facing page for public sharing.

Changes

• Adds a shared localized SEO metadata module.
• Adds metadata coverage for homepage, MeteorTest, Projects, and non-flagship project detail routes.
• Adds canonical URLs, alternate-language URLs, Open Graph metadata, and Twitter card metadata.
• Reshapes the MeteorTest page hero around Web preview, mock demo, GitHub, preview status, and private Local Agent boundary.
• Adds product-page styles for the MeteorTest hero, preview status card, and companion project panel.
• Updates English and Chinese site content for preview status messaging.
• Updates AGENTS.md and PROJECT_CONTENT_PLAN.md to mark Phase 11.2 complete and point next major work back to MeteorTest preview hardening.

Test Plan

• npm run lint
• npm run build
• npm run dev:local
• Verified http://127.0.0.1:3010/meteortest returns 200.
• Verified http://127.0.0.1:3010/zh-CN/meteortest returns 200.
• Verified the generated CSS resource returns 200.

Closes #36