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/temp-dir/index.d.ts

@@ -0,0 +1,18 @@
+/**
+Get the real path of the system temp directory.
+
+@example
+```
+import * as os from 'os';
+import tempDirectory = require('temp-dir');
+
+console.log(tempDirectory);
+//=> '/private/var/folders/3x/jf5977fn79jbglr7rk0tq4d00000gn/T'
+
+console.log(os.tmpdir());
+//=> '/var/folders/3x/jf5977fn79jbglr7rk0tq4d00000gn/T' // <= Symlink
+```
+*/
+declare const tempDirectory: string;
+
+export = tempDirectory;