Function resolve_backend_binary 

fn resolve_backend_binary(
    backend_name: &str,
    exe_suffix: &str,
) -> Result<PathBuf, String>

Resolve the backend binary path.

The CEF host lives in runtime/ alongside the backend binary in portable builds, so exe_dir IS the runtime directory. Search order:

1. Same dir as CEF host: {name}-{version}-{os}.{arch}.exe (versioned portable)
2. Same dir as CEF host: {name}.exe (dev mode — cargo build output)
3. Workspace dist/bin/: {name}-{version}-{os}.{arch}.exe
4. Workspace dist/bin/: {name}.exe (plain fallback)