feat(admin): add Chinese and English localization

[wandaocheng1997]

Summary

• Add Simplified Chinese and English localization across the complete admin application.
• Default the admin UI to Simplified Chinese, with language controls available before and after authentication.
• Persist and TypeBox-validate the locale preference while keeping published-site language separate.
• Add build-time literal extraction, route-chunk-local translations, a missing-message report, and an architecture coverage gate.
• Document the localization architecture and extension workflow.

Verification

• bun scripts/admin-i18n-report.ts --missing — 3,049 unique messages, 0 missing
• bun run build
• bun test — 6,579 passed, 0 failed
• bun run lint

Checklist

• Tests cover behavior changes.
• Documentation was updated.
• No compatibility shim was added.
• No deployment configuration, secrets, local databases, uploads, or generated artifacts are included.

[DavidBabinec]

Is this PR switching default admin UI to Chinese?

We appreciate the work, but please do not change default behaviour like that, this is breaking change for many people not speaking Chinese. Default admin UI language should remain English.

I will review the PR after tweaking it.

Thank you.