Complete Simplified Chinese scenario localization

[llwwbb]

Dependency

This draft PR is based on #5138 and includes its two commits. Please review and merge #5138 first; once it is merged, this PR's remaining diff will be the Simplified Chinese scenario localization commit.

Summary

• add a dedicated zh-cn UI locale without changing the existing Traditional Chinese zh locale
• translate all campaign, standalone, and supported homebrew scenario text into Simplified Chinese
• add general locale integrity checks for file/key coverage, locale-neutral keys, and English fallbacks

Test plan

• npm test
• npm run tc
• npx vite build

[llwwbb]

Now that #5138 has been merged, I have one question about card images for the new Simplified Chinese (zh-cn) locale.

Would you prefer zh-cn to reuse the existing Chinese (zh) card image pack? During playtesting, I found quite a few inaccuracies and mismatched card images in the current Chinese pack, so linking it by default may give Simplified Chinese users a confusing experience.

One option would be to keep zh-cn separate and fall back to the English card images for now, until a more reliable Simplified Chinese image pack is available. Do you have a preference, or a better idea for how this mapping should be handled?

[halogenandtoast]

Would you prefer zh-cn to reuse the existing Chinese (zh) card image pack? During playtesting, I found quite a few inaccuracies and mismatched card images in the current Chinese pack, so linking it by default may give Simplified Chinese users a confusing experience.

One option would be to keep zh-cn separate and fall back to the English card images for now, until a more reliable Simplified Chinese image pack is available. Do you have a preference, or a better idea for how this mapping should be handled?

I don't have enough language context to know which is best here. But I feel like falling back to English for now is probably best.