Support Chinese (zh-CN) localization for UI strings like "thinking"

When language is set to "zh-CN" in settings.json, the UI still displays English strings such as "thinking" and "thinking for Xs".

Expected behavior: These UI strings should be localized to Chinese, e.g. "思考中" / "思考了 X 秒".

Steps to reproduce:

1. Set "language": "zh-CN" in ~/.claude/settings.json
2. Observe that "thinking" status text remains in English

It would be great to have localized UI strings for at least zh-CN and other major languages.