fix: use platform-appropriate script path in downloader check on Windows

[natashaannn]

Fixes #242

How to test

On Windows

1. Clone the repo and run npm install
2. Run npx nodejs-whisper download before this fix (checkout main)
   • Expected: immediately throws [Nodejs-whisper] Error: Downloader not found.
3. Apply the fix (checkout this branch)
4. Run npx nodejs-whisper download again
   • Expected: model selection prompt appears, download proceeds, cmake build runs

Simulating the bug on any platform

The which() check can be tested in isolation:

const shell = require('shelljs')

// Simulates what the old code does on Windows
console.log(shell.which('./download-ggml-model.sh')) // null on Windows

// Simulates what the fix does on Windows
console.log(shell.which('download-ggml-model.cmd'))  // returns path on Windows

Run this inside cpp/whisper.cpp/models/ where the scripts live.

Unit-level check (any platform)

You can also force the Windows branch by temporarily hardcoding process.platform === 'win32' to true in the fixed code and confirming which() resolves download-ggml-model.cmd correctly.

Note

I later hit a libgomp-1.dll crash you hit as a downstream consequence of this bug: because the download/build never ran, an old MinGW-linked binary was sitting there from a prior partial build attempt. The real first failure was this which() check. Looking to fix this bug first before filing the libgomp one as a separate issue + PR after this one merges.

[ChetanXpro]

LGTM!