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/update-browserslist-db/check-npm-version.js

@@ -0,0 +1,17 @@
+let { execSync } = require('child_process')
+let pico = require('picocolors')
+
+try {
+  let version = parseInt(execSync('npm -v'))
+  if (version <= 6) {
+    process.stderr.write(
+      pico.red(
+        'Update npm or call ' +
+          pico.yellow('npx browserslist@latest --update-db') +
+          '\n'
+      )
+    )
+    process.exit(1)
+  }
+  // eslint-disable-next-line no-unused-vars
+} catch (e) {}