Add NixOS module and improve UI navigation and rendering

The flake now includes a NixOS module that enables integration with
system configuration. The TUI has been enhanced with better file
switching using arrow keys, space bar support for toggling values,
improved scrolling logic, styled category headings, and visual selection
feedback. Documentation comments were streamlined and placeholder files
removed.

flake.nix

@@ -23,7 +23,11 @@
     nixpkgs,
     naersk,
     fenix,
+    lib,
+    cfg,
   }: let
+    inherit (lib) mkOption mkIf types;
+
     system = "x86_64-linux";
     pkgs = nixpkgs.legacyPackages.${system};
 
@@ -32,10 +36,6 @@
       cargo = rustToolchain;
       rustc = rustToolchain;
     };
-
-    cargoToml = builtins.fromTOML (builtins.readFile ./Cargo.toml);
-    packageVersion = cargoToml.package.version;
-    # ...
   in {
     packages.${system} = {
       default =
@@ -44,11 +44,49 @@
       develop = naerskLib.buildPackage {
         name = "garandos-tui";
         src = ./.;
-        # buildInputs = with pkgs; [];
         nativeBuildInputs = with pkgs; [pkg-config];
       };
     };
 
+    nixosModules.garandos-tui = {
+      options.programs.garandos-tui = {
+        enable = mkOption {
+          type = types.bool;
+          default = false;
+          description = "Enable the Garandos TUI module";
+        };
+        package = mkOption {
+          type = types.package;
+          default = self.packages.${system}.default;
+          defaultText = "self.packages.${system}.default";
+          description = "The Garandos TUI package";
+        };
+        systemModulesFilePath = mkOption {
+          type = types.path;
+          default = "";
+          example = "/home/\${username}/garandos/hosts/\${hostname}/system-modules.nix";
+          description = "The path to the Host's system modules file";
+        };
+        homeModulesFilePath = mkOption {
+          type = types.path;
+          default = "";
+          example = "/home/\${username}/garandos/hosts/\${hostname}/home-modules.nix";
+          description = "The path to the Host's home modules file";
+        };
+      };
+      config = mkIf (cfg.enable && cfg.systemModulesFilePath != "" && cfg.homeModulesFilePath != "") {
+        environment.systemPackages = [
+          (pkgs.writeScriptBin "garandos-tui" ''
+            #!${pkgs.runtimeShell}
+            exec ${cfg.package}/bin/garandos_tui \
+              --sf '${cfg.systemModulesFilePath}' \
+              --hf '${cfg.homeModulesFilePath}' \
+              "$@"
+          '')
+        ];
+      };
+    };
+
     devShells.${system}.default = pkgs.mkShell {
       env.RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
       buildInputs = [
@@ -57,8 +95,6 @@
       nativeBuildInputs = with pkgs; [pkg-config];
 
       shellHook = ''
-        echo "garandos-tui    v${packageVersion}"
-        echo ""
         echo "Commands:"
         echo "    nix build            - Build production version"
         echo "    nix run              - Run production version"