feat: add Chinese localization

[GOSHEN192]

Summary

• Add persistent English/Chinese localization support.
• Move language selection to Settings > General.
• Translate safe static UI text and accessibility attributes while preserving user-generated content, code, project names, and agent output.
• Add localization tests.

Validation

• pnpm build
• pnpm test (26 files, 160 tests)
• git diff --check

Related discussion: #68

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