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/autoprefixer/lib/hacks/image-set.js

@@ -0,0 +1,18 @@
+let Value = require('../value')
+
+class ImageSet extends Value {
+  /**
+   * Use non-standard name for WebKit and Firefox
+   */
+  replace(string, prefix) {
+    let fixed = super.replace(string, prefix)
+    if (prefix === '-webkit-') {
+      fixed = fixed.replace(/("[^"]+"|'[^']+')(\s+\d+\w)/gi, 'url($1)$2')
+    }
+    return fixed
+  }
+}
+
+ImageSet.names = ['image-set']
+
+module.exports = ImageSet