feat: add Simplified Chinese UI localization

[fz999999]

Summary

• localize the main desktop UI into Simplified Chinese
• update window/app titles to match the Chinese UI
• default the auth import file picker to the current user's ~/.codex/auth.json

Verification

• corepack pnpm build
• corepack pnpm tauri build

Notes

• this change focuses on UI copy and import flow convenience
• no core account switching logic was changed beyond resolving the default auth file path for the file picker

[joonshakya]

Hi @fz999999

Seems like you completely replaced English texts with Chinese.
It would be great if there would be an option to switch languages.
Also the changes made have some conflicts with the latest main branch.
Can you refine these changes?