fix: use platform-aware URL opener on Windows

[fuxialexander]

Summary

• Fixes openUrl() in auth.ts to use cmd /c start on Windows instead of xdg-open
• xdg-open is a Linux-only utility and silently fails on Windows, preventing browser auto-open during device auth flow
• Keeps open for macOS and xdg-open for Linux

Root cause

The original code assumed non-macOS == Linux:

const cmd = platform() === 'darwin' ? 'open' : 'xdg-open'

xdg-open does not exist on Windows, so the browser never opens during login.

Fix

if (plt === 'darwin') {
  cmd = `open ${JSON.stringify(url)}`
} else if (plt === 'win32') {
  cmd = `cmd /c start "" ${JSON.stringify(url)}`
} else {
  cmd = `xdg-open ${JSON.stringify(url)}`
}

Test plan

• macOS: astro login still opens browser correctly
• Linux: astro login still opens browser correctly
• Windows: astro login now opens browser via cmd /c start

Closes astro-anywhere/astro-agent#130 (partial — the agent-runner has its own identical bug in setup.ts's tryOpenUrl() and start.ts's background spawn)

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