Integrate sendspin-cli as a custom package using python3.withPackages.
Packages aiosendspin from GitHub since it's only available in
nixpkgs-unstable. Includes all required dependencies: aiohttp, av,
numpy, qrcode, readchar, rich, sounddevice, and native libraries
(portaudio, ffmpeg).
Adds uv2nix flake inputs for future migration when sendspin-cli
adds a uv.lock file.
