Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/es-abstract/2025/SystemTimeZoneIdentifier.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var callBind = require('call-bind');
+
+var I402 = typeof Intl === 'undefined' ? null : Intl;
+var DateTimeFormat = I402 && I402.DateTimeFormat;
+var resolvedOptions = DateTimeFormat && callBind(DateTimeFormat.prototype.resolvedOptions);
+
+// https://262.ecma-international.org/15.0/#sec-systemtimezoneidentifier
+
+module.exports = function SystemTimeZoneIdentifier() {
+	if (DateTimeFormat && resolvedOptions) {
+		return resolvedOptions(new DateTimeFormat()).timeZone; // steps 2 - 3
+
+	}
+
+	return 'UTC'; // step 1
+};