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/@pmmmwh/react-refresh-webpack-plugin/client/ReactRefreshEntry.js

@@ -0,0 +1,23 @@
+/* global __react_refresh_library__ */
+
+const safeThis = require('core-js-pure/features/global-this');
+const RefreshRuntime = require('react-refresh/runtime');
+
+if (process.env.NODE_ENV !== 'production') {
+  if (typeof safeThis !== 'undefined') {
+    var $RefreshInjected$ = '__reactRefreshInjected';
+    // Namespace the injected flag (if necessary) for monorepo compatibility
+    if (typeof __react_refresh_library__ !== 'undefined' && __react_refresh_library__) {
+      $RefreshInjected$ += '_' + __react_refresh_library__;
+    }
+
+    // Only inject the runtime if it hasn't been injected
+    if (!safeThis[$RefreshInjected$]) {
+      // Inject refresh runtime into global scope
+      RefreshRuntime.injectIntoGlobalHook(safeThis);
+
+      // Mark the runtime as injected to prevent double-injection
+      safeThis[$RefreshInjected$] = true;
+    }
+  }
+}