feat(i18n): add Simplified Chinese to the interface and prompt languages

[omartelo]

Adds Simplified Chinese (zh-CN, shown as 简体中文) as an interface language and a prompt language. Any Chinese browser language maps to it.

• Frontend: locales/zh-cn/* covers every namespace, typed satisfies Shape<...>.
• Backend: internal/prompt/zh_cn.go is a full Catalog. Chinese has no plural, so isOneChinese is never true and Pick always returns Other. LICH_PROMPT_LANG=zh-CN.
• Literal in every locale: [lich], tool and CLI names, placeholders, format verbs and key names. The spawn briefing stays cmd.exe-safe; the per-Langs test covers it.
• Tests: zh-CN is added to the locale map and to the Parse/plural tables. New render tests on both sides. No existing assertion changed.
• Rebased on feat(i18n): add Spanish to the interface and prompt languages #731 (Spanish). The language lists now hold both, in the order en, pt-BR, es, zh-CN.

lich-plugin already ships the zh-CN mod catalog (lichdotdev/lich-plugin#71); it reaches users with the next plugin release.