docs: add Simplified Chinese README

[whtis]

What

• Add README.zh-CN.md — a Simplified Chinese translation of the project README.
• Add a small English | 简体中文 language switcher at the top of the English README.md.

Why

Rowboat has growing visibility globally, including in Mandarin-speaking developer communities, but the project currently only ships an English README. A native Chinese version lowers the barrier for users who want to evaluate Rowboat without translating the README themselves.

Notes on the translation

• Mirrors the structure and tone of the English README — no scope changes, no marketing edits.
• Keeps product names, file paths, badges, and external links untouched.
• Uses idiomatic developer-oriented Chinese, not a machine-translation pass.
• Original concepts that are typically left in English in Chinese tech writing (local-first, Live notes, Bring your own model, MCP) are preserved alongside the Chinese phrasing for clarity.

How tested

• Rendered both files locally to confirm the Markdown structure, badges, and image embeds resolve correctly.
• Verified the English | 简体中文 link in README.md points to ./README.zh-CN.md and back.
• No source code touched, so no test suite run is applicable.

Drafted with AI assistance; verified by author before publishing.