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.