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/at-least-node/index.js

@@ -0,0 +1,5 @@
+module.exports = r => {
+  const n = process.versions.node.split('.').map(x => parseInt(x, 10))
+  r = r.split('.').map(x => parseInt(x, 10))
+  return n[0] > r[0] || (n[0] === r[0] && (n[1] > r[1] || (n[1] === r[1] && n[2] >= r[2])))
+}