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

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Maintenance triage — 2026-09-30

Continue in: #181.

Simplified Chinese localization is consolidated in #157 / #181, which keeps English, language selection, fallback behavior, and native-menu coverage. The optional default auth-file-picker path enhancement can return as a separate focused change with a fallback when path resolution fails. #181 remains open; localization is not yet released.

[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?

[Lampese]

Requesting changes for two product-level issues:

1. This PR does not add Simplified Chinese as an additional localization layer; it replaces the only existing UI language with Chinese. The document language, window title, product name, and in-app copy are all switched to hard-coded Chinese without any locale selection, detection, or fallback. That is a regression for current non-Chinese users rather than a new localization capability.

2. The new default auth.json path flow makes file import more fragile. handleSelectFile now depends on a backend call to resolve the default path first, and if that command fails, the file dialog is never opened at all. Previously, users could still browse manually even without a default path. The backend also converts the resolved path through to_str(), which can fail on non-UTF-8 filesystem paths and trigger this regression unnecessarily.

I think both issues should be resolved before merging. The localization part should preserve an English fallback or proper locale switching, and the import picker should still open even if resolving the default path fails.