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/fork-ts-checker-webpack-plugin/lib/profile/Performance.d.ts

@@ -0,0 +1,11 @@
+interface Performance {
+    enable(): void;
+    disable(): void;
+    mark(name: string): void;
+    markStart(name: string): void;
+    markEnd(name: string): void;
+    measure(name: string, startMark?: string, endMark?: string): void;
+    print(): void;
+}
+declare function createPerformance(): Performance;
+export { Performance, createPerformance };