feat(i18n): add Simplified Chinese localization

[mydanyi]

Summary

• add a complete Simplified Chinese (zh) catalog
• map the current Steam/Armada zh-cn locale to zh (in addition to schinese)
• localize previously hardcoded FP16, installation, profile, toast, Flatpak, and diagnostics UI strings
• regenerate src/i18n/languages.json

This is a clean alternative to #250. That PR's source defaults/i18n/zh.json contains invalid JSON because of unescaped quotes, and it does not cover several user-visible strings or the zh-cn locale reported by current Armada builds.

Validation

• pnpm run build completed successfully
• all i18n JSON files parse successfully
• template and Chinese catalogs both contain 156 keys, with no missing or extra keys
• every t(...) call has a matching template and Chinese key
• tested on Armada OS 20260822.a8dabdd, Decky Loader v3.2.8-pre1, and Decky LSFG-VK v0.12.8 on an AArch64 handheld
• verified the live Quick Access UI uses locale zh-cn and displays translated installation status, usage instructions, Flatpak setup, and diagnostics modal

The deployed test changed only dist/index.js; the packaged ARM64 library remained untouched and retained its original SHA256.