Add standalone Windows build and GitHub Actions pipeline

[abhinavwh17]

Summary

Added support for packaging NexaMind as a standalone desktop application and generating a downloadable Windows build through GitHub Actions.

Changes

• Added launcher.py to:
• Start the local FastAPI server.
• Serve NexaMind on 127.0.0.1:8000.
• Automatically open NexaMind in Google Chrome.
• Fall back to the default browser if Chrome is unavailable.
• Updated FastAPI to serve the production React/Vite build.
• Updated frontend production configuration to use the bundled local backend.
• Added NexaMind.spec for packaging NexaMind using PyInstaller.
• Bundled the React frontend/dist output inside the standalone application.
• Included required keyring backends in the packaged application.
• Added PyInstaller and required packaging dependencies.
• Added GitHub Actions Windows build workflow.

Pipeline now:

• Installs Node dependencies.
• Builds the React frontend.
• Installs Python dependencies.
• Builds NexaMind.exe using PyInstaller.
• Packages the executable as NexaMind-Windows.zip.
• Publishes the ZIP as a GitHub Actions artifact.
• Updated .gitignore to exclude generated build directories and local development files.

Result

Users can download the generated Windows and macOS artifact, extract it, and run the exe or dmg file