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/minimalistic-assert/index.js

@@ -0,0 +1,11 @@
+module.exports = assert;
+
+function assert(val, msg) {
+  if (!val)
+    throw new Error(msg || 'Assertion failed');
+}
+
+assert.equal = function assertEqual(l, r, msg) {
+  if (l != r)
+    throw new Error(msg || ('Assertion failed: ' + l + ' != ' + r));
+};