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/web-vitals/dist/modules/lib/polyfills/firstInputPolyfill.d.ts

@@ -0,0 +1,7 @@
+import { FirstInputPolyfillCallback } from '../../types.js';
+/**
+ * Accepts a callback to be invoked once the first input delay and event
+ * are known.
+ */
+export declare const firstInputPolyfill: (onFirstInput: FirstInputPolyfillCallback) => void;
+export declare const resetFirstInputPolyfill: () => void;