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/cjs/_sequence.js

@@ -0,0 +1,14 @@
+function sequence(iteratee, start, stop, step) {
+  if (stop == null) {
+    stop = start || 0;
+    start = 0;
+  }
+  if (!step) {
+    step = stop < start ? -1 : 1;
+  }
+  var rest = (stop - start) % step;
+  stop += (rest && step - rest);
+  for ( ; start != stop ; start += step) if (iteratee(start)) return start;
+}
+
+module.exports = sequence;