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/@testing-library/jest-dom/dist/to-have-focus.js

@@ -0,0 +1,16 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.toHaveFocus = toHaveFocus;
+var _utils = require("./utils");
+function toHaveFocus(element) {
+  (0, _utils.checkHtmlElement)(element, toHaveFocus, this);
+  return {
+    pass: element.ownerDocument.activeElement === element,
+    message: () => {
+      return [this.utils.matcherHint(`${this.isNot ? '.not' : ''}.toHaveFocus`, 'element', ''), '', ...(this.isNot ? ['Received element is focused:', `  ${this.utils.printReceived(element)}`] : ['Expected element with focus:', `  ${this.utils.printExpected(element)}`, 'Received element with focus:', `  ${this.utils.printReceived(element.ownerDocument.activeElement)}`])].join('\n');
+    }
+  };
+}