i18n: wrap hardcoded UI strings for Chinese localization

[seldoms]

Summary

• Wrap remaining hardcoded UI labels and buttons with tr()/tl!() calls
• Add missing Simplified and Traditional Chinese translations
• Add entries to other language catalogs for consistency

Test plan

• cargo test -p vectorcraft-ui-egui --lib i18n passes
• UI strings display in Simplified Chinese when macOS preferred language is Chinese

[bflatastic]

Thanks, @seldoms, and welcome! Merged after a full CI run. The newly wrapped strings, the catalogue rows and the C/POSIX locale handling all went in. One change on top (22443d8): I dropped the system CJK font loaded in theme::install_fonts. The app already does this in ui_fonts: the first time a frame paints a CJK character the UI fonts lack, it finds an installed font that covers it (in the background) and adds it as the last fallback. Loading one at every startup duplicated that, cost a large font read for interfaces that never show CJK, and broke that module's test. With your change in, Chinese text in the UI still gets PingFang, YaHei or Noto through that path.

We'd love to have you on our Discord too: https://discord.gg/artcraft