fix(init): detect native system language

[whitelonng]

What changed

• Detect macOS language from AppleLanguages, with AppleLocale fallback.
• Detect Windows UI culture through a direct powershell.exe invocation.
• Read Linux locale variables including LANGUAGE, LC_MESSAGES, and LANG.
• Treat C, C.UTF-8, and POSIX as unknown while keeping --lang as the highest-priority override.
• Add regression coverage for macOS, Windows, and Linux locale selection.
• Make the Windows smoke workflow verify native UI culture wins over deliberately conflicting terminal locale variables in CMD, PowerShell, and Git Bash.

Why

On a Chinese macOS system, terminal variables could report C.UTF-8 and Node could report en-US, causing mancode init to display English. Windows Git Bash could have a similar mismatch between shell locale and the Windows UI language.

Impact

Initialization now follows the computer UI language reliably on macOS and Windows, while Linux follows its standard locale environment. Explicit --lang zh-CN|en behavior is unchanged.

Validation

• npm run lint
• npm run typecheck
• npm run build
• npm test (401 tests)
• npm run test:windows-smoke
• Real Chinese macOS run with LC_ALL=C.UTF-8
• git diff --check