Add sendspin-cli package with Python dependencies

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.

packages/default.nix

@@ -1,7 +1,8 @@
-{ pkgs, ... }:
+{ pkgs, uv2nix ? null, pyproject-nix ? null, pyproject-build-systems ? null, ... }:
 {
   vulkanHDRLayer = pkgs.callPackage ./vulkan-hdr-layer {};
   tea-rbw = pkgs.callPackage ./tea-rbw {};
   app-launcher-server = pkgs.callPackage ./app-launcher-server {};
   claude-code = pkgs.callPackage ./claude-code {};
+  sendspin-cli = pkgs.callPackage ./sendspin-cli { inherit uv2nix pyproject-nix pyproject-build-systems; };
 }