fix(viewer): make Chinese locale selectable

[wbugitlab1]

Fixes #288

Summary

• add an in-viewer English/Chinese language selector with local persistence
• expose bundled viewer locales to browser code and auto-select Chinese browser preferences when no viewer preference exists
• localize representative Dashboard and Memories surfaces, including empty states, table headers, and delete confirmation copy
• add Chinese CJK system font fallbacks and document VIEWER_LANGUAGE=zh / zh-CN
• stabilize the Codex SDK provider test timeout so the exact full suite stays green under parallel load

Verification

• corepack pnpm run build
• corepack pnpm test
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact
• gitleaks detect --source . --redact --log-opts origin/main..HEAD
• gitleaks detect --source . --redact --no-git