fix(installer): correct npm global bin path on Windows

[openperf]

Summary

This PR fixes an issue in the PowerShell installer (public/install.ps1) where the incorrect path for npm's global binaries was being added to the user's PATH on Windows.

• On Windows, npm config get prefix returns the directory where executables are located (e.g., %APPDATA%\npm), not a parent directory. The script was incorrectly appending \bin to this path, a convention that only applies to Unix-like systems.
• This change corrects the logic to use the npm config get prefix output directly as the binary path, ensuring openclaw is found after installation.
• The Expected path: message shown to the user on failure has also been corrected to reflect the actual path without the \bin suffix.

Test Plan

This change can be manually verified on a Windows machine:

1. Run the modified installer script: iwr -useb <your-raw-script-url> | iex
2. Observe the output. It should now correctly identify and add the ...\npm directory to the PATH.
3. After restarting the terminal, openclaw --version should execute successfully.

Fixes openclaw/openclaw#19921

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