fix: use nssm for windows service

[pixelcola]

Summary

• replace direct sc.exe service management with NSSM commands
• configure NSSM Application and AppParameters before starting sing-box
• document NSSM PATH requirement and update backend specs

Tests

• env UV_CACHE_DIR=/tmp/uv-cache uv run ruff check src tests
• env UV_CACHE_DIR=/tmp/uv-cache uv run ty check src
• env UV_CACHE_DIR=/tmp/uv-cache uv run pytest