docs: add UI specification guidance

[horizonx-so]

Summary

• Extends the Simplified Chinese and English Spec Coding guides with UI-specific constraints that make visual intent testable
• Covers design tokens, complete component states, responsive behavior, accessibility, motion, and production-readiness gates
• Links one free, vendor-neutral field model as an optional example, not a required tool

Validation

• Checked the repository guidelines, current localized guides, recent merged documentation contributions, and duplicate state
• Kept the change to one contextual paragraph in each of the zh-cn and en files
• No existing HorizonX or Vibe Coding UI Specification entry was found

Disclosure

I founded HorizonX, which publishes and commercially maintains the linked free specification and also offers paid UI/code products. Research, drafting, translation alignment, and verification were AI-assisted. Please include the reference only if it independently improves the course. There is no payment, sponsorship, affiliate arrangement, reciprocity, guarantee, or preferred anchor request. Contact: support@horizonx.so.