Add Simplified Chinese localization

[Luthics]

Summary

• add zh-CN as a selectable account locale
• add complete Simplified Chinese translations for the Rails UI
• add complete Simplified Chinese translations for the signing form and template builder
• add Chinese date and time formats while preserving interpolation, HTML, and URL tokens

Context

This updates and narrows the Simplified Chinese work from #677 by @new985211 to the current 3.2.0 codebase. That PR was closed because it also changed Docker and Compose build files; this PR intentionally changes only the five locale integration files and contains no deployment or build changes.

Addresses #535.

Validation

• Rails locale parity: 1067/1067 keys, with matching placeholders
• signing form parity: 116/116 keys, with matching placeholders
• template builder parity: 241/241 keys, with matching placeholders
• YAML parse and standalone I18n load, interpolation, and Chinese date-format checks
• ESLint on both JavaScript locale files
• RuboCop on the Ruby integration files
• git diff --check