add packages.default flake output

this likely doesn't work (yet),
cannot properly test because at time of writing,
the app doesn't compile through `flutter run` either.

flake.nix

@@ -29,6 +29,20 @@
           ...
         }:
 
+        let
+          src = ./.;
+
+          # from https://discourse.nixos.org/t/is-there-a-way-to-read-a-yaml-file-and-get-back-a-set/18385/5
+          fromYAML =
+            file:
+            builtins.fromJSON (
+              builtins.readFile (
+                pkgs.runCommand "converted-yaml.json" { } ''${pkgs.yj}/bin/yj < "${file}" > "$out"''
+              )
+            );
+
+          package = fromYAML "${src}/pubspec.yaml";
+        in
         {
           _module.args.pkgs = import nixpkgs {
             inherit system;
@@ -59,6 +73,20 @@
               CPATH = lib.makeSearchPath "include" [ pkgs.glibc.dev ];
             };
           };
+
+          packages.default = pkgs.flutter.buildFlutterApplication {
+            inherit src;
+            pname = package.name;
+            version = package.version;
+
+            pubspecLock = fromYAML "${src}/pubspec.lock";
+            gitHashes = {
+              flutter_chat_ui = "sha256-4fuag7lRH5cMBFD3fUzj2K541JwXLoz8HF/4OMr3uhk=";
+              flutter_link_previewer = "sha256-4fuag7lRH5cMBFD3fUzj2K541JwXLoz8HF/4OMr3uhk=";
+              flyer_chat_text_message = "sha256-4fuag7lRH5cMBFD3fUzj2K541JwXLoz8HF/4OMr3uhk=";
+              window_size = "sha256-XelNtp7tpZ91QCEcvewVphNUtgQX7xrp5QP0oFo6DgM=";
+            };
+          };
         };
     };
 }