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/core-js-pure/internals/same-value-zero.js

@@ -0,0 +1,7 @@
+'use strict';
+// `SameValueZero` abstract operation
+// https://tc39.es/ecma262/#sec-samevaluezero
+module.exports = function (x, y) {
+  // eslint-disable-next-line no-self-compare -- NaN check
+  return x === y || x !== x && y !== y;
+};