docs(readme): add Windows desktop installer links and update download badge

[md-abusayeed]

Problem / Motivation

The main README.md previously indicated that Windows had no desktop build available (- **Windows**: no desktop build yet, so run the Gateway from a source install), and the download badge at the top omitted Windows (Download - macOS | Linux).

Following the addition and validation of the Windows desktop packaging workflows (build-windows.yml, publish-windows.yml), the Authenticode-signed NSIS desktop installer (KiroCrew-Setup.exe) is actively published to the distribution CDN on the insider and nightly channels.

Why it matters

Windows users reading the README were guided exclusively toward running from source even though official signed desktop installer packages are actively available for download.

What changed

• Updated the download badge in README.md to Download - macOS | Linux | Windows.
• Updated the "App downloads" section in README.md to include direct download links for the Windows desktop installer on the Insider and Nightly channels alongside macOS and Linux.

Tests

• Verified markdown formatting and links.

Manual verification

• Verified URL targets match the standard CDN pattern (https://download.crew.kiro.dev/desktop/<channel>/latest/KiroCrew-Setup.exe).

Screenshots / video

Why no screenshot: documentation / link update in README.md.

Related Issues

None.

Checklist

• Single commit with a Conventional Commits title (feat|fix|docs|refactor|perf|test|chore|ci|build|revert: ...)
• Existing tests pass and new tests added for new functionality
• Self-review completed; code follows project style guidelines
• Documentation updated (if applicable)
• No secrets, credentials, or internal references in the diff