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/getFirstHiddenTimePolyfill.js

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+let firstHiddenTime = document.visibilityState === 'hidden' ? 0 : Infinity;
+const onVisibilityChange = (event) => {
+    if (document.visibilityState === 'hidden') {
+        firstHiddenTime = event.timeStamp;
+        removeEventListener('visibilitychange', onVisibilityChange, true);
+    }
+};
+// Note: do not add event listeners unconditionally (outside of polyfills).
+addEventListener('visibilitychange', onVisibilityChange, true);
+export const getFirstHiddenTime = () => firstHiddenTime;