fix(i18n): fall back when request language is missing

[FenjuFu]

Summary

• fall back to the built-in zh-CN translation map when a request has no supported language
• prevent SSR pages, the root layout, and metadata generation from failing for probes, crawlers, and clients without Accept-Language

Root cause

mobx-i18n returns undefined when neither the language cookie nor Accept-Language matches a configured language. The app destructured that result unconditionally, so every page render threw before producing HTML.

Impact

Requests without a supported language now render in Simplified Chinese instead of returning HTTP 500. Existing zh-CN, zh-TW, and en-US negotiation remains unchanged.

Verification

• pnpm exec next build
• pnpm exec prettier --write i18n/index.ts
• pnpm exec eslint i18n/index.ts
• pnpm exec tsc --noEmit
• production server requests to /home: no language header -> 200 / zh-CN; zh-CN -> 200 / zh-CN; en-US -> 200 / en-US

[github-actions]

Cloudflare Preview Deployment

• Branch: agent/fix-ssr-language-fallback
• Commit: 669ce7c
• Preview Alias: b-agent-fix-ssr-language-fallback

Wrangler Output

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Worker Version ID: bed385c3-55d0-4fcd-8754-ed2196a22808
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Version Preview Alias URL: https://b-agent-fix-ssr-language-fallback-goodaction-hub.shiy2008.workers.dev