feat(web_adapter): implement download method for web platforms

[meylis1998]

Summary

• Implement the download() method for web platforms using the Blob API instead of throwing UnsupportedError
• Add comprehensive tests for the new functionality
• Update CHANGELOG

Details

This PR enables file downloads in web browsers using the same dio.download() API that works on native platforms.

How it works:

1. Fetches file content as bytes with full progress tracking support
2. Creates a Blob from the response bytes
3. Triggers the browser's native download dialog via an anchor element click
4. Uses savePath as the suggested filename (extracts filename portion from paths)

Supported features on web:

Feature	Support
onReceiveProgress	✅ Full support
cancelToken	✅ During fetch phase
String savePath	✅ Used as suggested filename
Callback savePath	✅ Receives headers, returns filename
Custom headers/auth	✅ Full support

Limitations (documented in code):

• deleteOnError parameter is ignored (not applicable on web)
• fileAccessMode parameter is ignored (not applicable on web)
• Entire file is loaded into memory before download triggers (not suitable for very large files)
• Actual save location is determined by browser settings, not by savePath

Usage example:

// Works the same on web now!
await dio.download(
  'https://example.com/file.pdf',
  'my-file.pdf',  // Used as suggested filename
  onReceiveProgress: (received, total) {
    print('${(received / total * 100).toStringAsFixed(0)}%');
  },
);

Fixes #2408

Test plan

• dart analyze passes with no issues
• Added unit tests for argument validation
• Added tests for String and callback savePath variants
• Added tests for progress callback and cancellation support
• Manual testing in browser environment