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/chain.js

@@ -0,0 +1,12 @@
+define(['./underscore'], function (underscore) {
+
+  // Start chaining a wrapped Underscore object.
+  function chain(obj) {
+    var instance = underscore(obj);
+    instance._chain = true;
+    return instance;
+  }
+
+  return chain;
+
+});