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/fastq/example.mjs

@@ -0,0 +1,11 @@
+import { promise as queueAsPromised } from './queue.js'
+
+/* eslint-disable */
+
+const queue = queueAsPromised(worker, 1)
+
+console.log('the result is', await queue.push(42))
+
+async function worker (arg) {
+  return 42 * 2
+}