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.

flake.lock

@@ -256,6 +256,52 @@
         "type": "github"
       }
     },
+    "pyproject-build-systems": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "pyproject-nix": [
+          "pyproject-nix"
+        ],
+        "uv2nix": [
+          "uv2nix"
+        ]
+      },
+      "locked": {
+        "lastModified": 1763662255,
+        "narHash": "sha256-4bocaOyLa3AfiS8KrWjZQYu+IAta05u3gYZzZ6zXbT0=",
+        "owner": "pyproject-nix",
+        "repo": "build-system-pkgs",
+        "rev": "042904167604c681a090c07eb6967b4dd4dae88c",
+        "type": "github"
+      },
+      "original": {
+        "owner": "pyproject-nix",
+        "repo": "build-system-pkgs",
+        "type": "github"
+      }
+    },
+    "pyproject-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1764134915,
+        "narHash": "sha256-xaKvtPx6YAnA3HQVp5LwyYG1MaN4LLehpQI8xEdBvBY=",
+        "owner": "pyproject-nix",
+        "repo": "pyproject.nix",
+        "rev": "2c8df1383b32e5443c921f61224b198a2282a657",
+        "type": "github"
+      },
+      "original": {
+        "owner": "pyproject-nix",
+        "repo": "pyproject.nix",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "google-cookie-retrieval": "google-cookie-retrieval",
@@ -267,7 +313,33 @@
         "nixpkgs": "nixpkgs_2",
         "nixpkgs-unstable": "nixpkgs-unstable",
         "plasma-manager": "plasma-manager",
-        "plasma-manager-unstable": "plasma-manager-unstable"
+        "plasma-manager-unstable": "plasma-manager-unstable",
+        "pyproject-build-systems": "pyproject-build-systems",
+        "pyproject-nix": "pyproject-nix",
+        "uv2nix": "uv2nix"
+      }
+    },
+    "uv2nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "pyproject-nix": [
+          "pyproject-nix"
+        ]
+      },
+      "locked": {
+        "lastModified": 1766021660,
+        "narHash": "sha256-UUfz7qWB1Rb2KjGVCimt//Jncv3TgJwffPqbzqpkmgY=",
+        "owner": "pyproject-nix",
+        "repo": "uv2nix",
+        "rev": "19fa99be3409f55ec05e823c66c9769df7a8dd17",
+        "type": "github"
+      },
+      "original": {
+        "owner": "pyproject-nix",
+        "repo": "uv2nix",
+        "type": "github"
       }
     }
   },