Translate README to Chinese (README.zh.md)

[Jiseoup]

Context

The README is available in English (README.md) and Korean (README.ko.md).
This issue adds a Chinese translation, README.zh.md.

Beginner-friendly — no code, just translation following the existing pattern.
It's larger than a UI-string task, so take your time.

What to do

1. Create README.zh.md by translating README.md into Simplified Chinese. Keep the same structure, headings, tables, images, and links.

2. Update the language switcher at the top of every README file so each lists all available languages (bold = current language, others linked). In README.zh.md, Chinese is bold.

   Example switcher for README.zh.md:

   <a href="README.md">English</a> | <a href="README.ko.md">한국어</a> | <b>中文</b>

   And add | <a href="README.zh.md">中文</a> to the switcher in README.md and README.ko.md.

Keep as-is (do not translate)

• Code blocks, commands, env var names, and file paths.
• The "Deploy with Vercel" button and URLs.
• GitHub status badge labels (Private/Public, etc.) if referenced.

Acceptance criteria

• README.zh.md mirrors README.md in structure and content
• Language switcher updated in all README files; all links resolve
• npm run format:check passes (Prettier formats Markdown)

Comment below to claim this — we'll assign it to avoid duplicate work.