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/underscore/amd/_cb.js

@@ -0,0 +1,12 @@
+define(['./underscore', './_baseIteratee', './iteratee'], function (underscore, _baseIteratee, iteratee) {
+
+  // The function we call internally to generate a callback. It invokes
+  // `_.iteratee` if overridden, otherwise `baseIteratee`.
+  function cb(value, context, argCount) {
+    if (underscore.iteratee !== iteratee) return underscore.iteratee(value, context);
+    return _baseIteratee(value, context, argCount);
+  }
+
+  return cb;
+
+});