[codex] Add admin interface localization

[wolone]

Summary

• add a lightweight localization layer with English and Simplified Chinese catalogs
• detect and persist the preferred locale
• add language switching to full-screen flows and the authenticated sidebar
• localize navigation, page titles, login, and the sites overview

Why

Pilot's admin interface currently exposes hard-coded English copy. This establishes a small dependency-free localization foundation and delivers an initial translated user flow without changing backend behavior.

Validation

• npm run build with Node.js 24.18.0
• 42 focused backend tests passed
• 6 Node-dependent backend tests passed
• locale catalogs contain matching keys
• git diff --check

[greptile-apps]

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

_{Reviews (2): Last reviewed commit: "fix: refresh pending toast translations" | Re-trigger Greptile}