feat(docs): Simplified Chinese + Japanese translations

[vishr]

Full Simplified Chinese (zh-cn) and Japanese (ja) translations of the docs, generated via Codex against a fixed spec + glossary, and independently verified.

What's included

• i18n config — root locale (English stays at /…) + zh-cn + ja, with translated sidebar group labels (keyed by BCP-47 lang tag, e.g. zh-CN).
• Translated content — every page in Guide (14), Middleware (25), Cookbook (20) → 59 pages × 2 = 118 files.
• Starlight's built-in UI (search, TOC, nav, theme/lang pickers) auto-translates for both locales.

Translation rules applied

Prose + frontmatter title/description translated; code, identifiers, import paths, commands, header names kept verbatim; asides preserved; in-locale links prefixed (/zh-cn/…, /ja/…); a shared glossary (middleware→中间件/ミドルウェア, context→上下文/コンテキスト, etc.).

Verification (all pass)

• npm run build → 181 pages, no errors, all locales generated.
• English sources untouched (additions only).
• echo.Context preserved in 46 files/lang; /v5 imports in 29; 0 unprefixed in-locale link leaks; 0 pages missing description; glossary consistent (中间件 ×38, ミドルウェア ×38).
• Prose spot-checked in both languages — natural and accurate.

Not included

• Homepage (index.mdx) stays English (custom marketing MDX — separate task).
• Maintenance note: future English doc edits leave zh-cn/ja stale until re-run.

🤖 Generated with Claude Code