Windows (PowerShell) setup instructions to the "Using a virtual environment (Recommended)" section in CONTRIBUTING.md

[fuahyo]

What Changed

Added Windows (PowerShell) setup instructions to the "Using a virtual environment (Recommended)" section in CONTRIBUTING.md.

Why

The contributing guide previously only included Linux/macOS commands for setting up a virtual environment. This made it difficult for Windows users to get started with local development.

Changes

• Added PowerShell commands for creating and activating virtual environments on Windows
• Included a helpful note about execution policy settings that may be needed for PowerShell activation
• Maintained consistency with the existing Linux example format

Testing

• Verified PowerShell commands work on Windows 10/11
• Confirmed the execution policy note addresses common activation issues

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