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/user-event/dist/utils/focus/selector.js

@@ -0,0 +1,8 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.FOCUSABLE_SELECTOR = void 0;
+const FOCUSABLE_SELECTOR = ['input:not([type=hidden]):not([disabled])', 'button:not([disabled])', 'select:not([disabled])', 'textarea:not([disabled])', '[contenteditable=""]', '[contenteditable="true"]', 'a[href]', '[tabindex]:not([disabled])'].join(', ');
+exports.FOCUSABLE_SELECTOR = FOCUSABLE_SELECTOR;