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/observe.d.ts

@@ -0,0 +1,12 @@
+export interface PerformanceEntryHandler {
+    (entry: PerformanceEntry): void;
+}
+/**
+ * Takes a performance entry type and a callback function, and creates a
+ * `PerformanceObserver` instance that will observe the specified entry type
+ * with buffering enabled and call the callback _for each entry_.
+ *
+ * This function also feature-detects entry support and wraps the logic in a
+ * try/catch to avoid errors in unsupporting browsers.
+ */
+export declare const observe: (type: string, callback: PerformanceEntryHandler) => PerformanceObserver | undefined;