Website restoration: Node.js backend, restored design, GitHub sync disabled

- Replaced broken .NET backend with Node.js/Express
- Restored original website design with purple gradient hero
- Updated homepage and shop pages with Bootstrap 5 responsive design
- Disabled GitHub remote sync - all code now stored locally only
- Created backup scripts and documentation
- All changes saved on Ubuntu server at /var/www/SkyArtShop

backend/node_modules/math-intrinsics/sign.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var $isNaN = require('./isNaN');
+
+/** @type {import('./sign')} */
+module.exports = function sign(number) {
+	if ($isNaN(number) || number === 0) {
+		return number;
+	}
+	return number < 0 ? -1 : +1;
+};