satty: add satty to program modules

Add satty, a wlroots based screenshot annotation tool, to the programs
module.

modules/programs/satty.nix

@@ -0,0 +1,57 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.programs.satty;
+
+  tomlFormat = pkgs.formats.toml { };
+in
+{
+  meta.maintainers = [ lib.hm.maintainers.gauthsvenkat ];
+
+  options.programs.satty = {
+    enable = lib.mkEnableOption "Satty - Modern Screenshot Annotation";
+
+    package = lib.mkPackageOption pkgs "satty" { nullable = true; };
+
+    settings = lib.mkOption {
+      type = tomlFormat.type;
+      default = { };
+      example = lib.literalExpression ''
+        {
+          general = {
+            fullscreen = true;
+            corner-roundness = 12;
+            initial-tool = "brush";
+            output-filename = "/tmp/test-%Y-%m-%d_%H:%M:%S.png";
+          };
+          color-palette = {
+            palette = [ "#00ffff" "#a52a2a" "#dc143c" "#ff1493" "#ffd700" "#008000" ];
+          };
+        }
+      '';
+      description = ''
+        Configuration for Satty written to {file}`$XDG_CONFIG_HOME/satty/config.toml`.
+
+        See the [Satty documentation](https://github.com/gabm/Satty#configuration-file)
+        for available options.
+      '';
+    };
+
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      (lib.hm.assertions.assertPlatform "programs.satty" pkgs lib.platforms.linux)
+    ];
+
+    home.packages = lib.mkIf (cfg.package != null) [ cfg.package ];
+
+    xdg.configFile."satty/config.toml" = lib.mkIf (cfg.settings != { }) {
+      source = tomlFormat.generate "satty-config.toml" cfg.settings;
+    };
+  };
+}