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/promise/polyfill-done.js

@@ -0,0 +1,12 @@
+// should work in any browser without browserify
+
+if (typeof Promise.prototype.done !== 'function') {
+  Promise.prototype.done = function (onFulfilled, onRejected) {
+    var self = arguments.length ? this.then.apply(this, arguments) : this
+    self.then(null, function (err) {
+      setTimeout(function () {
+        throw err
+      }, 0)
+    })
+  }
+}