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/background-size.js

@@ -0,0 +1,23 @@
+let Declaration = require('../declaration')
+
+class BackgroundSize extends Declaration {
+  /**
+   * Duplication parameter for -webkit- browsers
+   */
+  set(decl, prefix) {
+    let value = decl.value.toLowerCase()
+    if (
+      prefix === '-webkit-' &&
+      !value.includes(' ') &&
+      value !== 'contain' &&
+      value !== 'cover'
+    ) {
+      decl.value = decl.value + ' ' + decl.value
+    }
+    return super.set(decl, prefix)
+  }
+}
+
+BackgroundSize.names = ['background-size']
+
+module.exports = BackgroundSize