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/modules/templateSettings.js

@@ -0,0 +1,9 @@
+import _ from './underscore.js';
+
+// By default, Underscore uses ERB-style template delimiters. Change the
+// following template settings to use alternative delimiters.
+export default _.templateSettings = {
+  evaluate: /<%([\s\S]+?)%>/g,
+  interpolate: /<%=([\s\S]+?)%>/g,
+  escape: /<%-([\s\S]+?)%>/g
+};