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/eslint-plugin-jsx-a11y/docs/rules/tabindex-no-positive.md

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+# jsx-a11y/tabindex-no-positive
+
+💼 This rule is enabled in the following configs: ☑️ `recommended`, 🔒 `strict`.
+
+<!-- end auto-generated rule header -->
+
+Avoid positive `tabIndex` property values to synchronize the flow of the page with keyboard tab order.
+
+## Rule details
+
+This rule takes no arguments.
+
+### Succeed
+```jsx
+<span tabIndex="0">foo</span>
+<span tabIndex="-1">bar</span>
+<span tabIndex={0}>baz</span>
+```
+
+### Fail
+```jsx
+<span tabIndex="5">foo</span>
+<span tabIndex="3">bar</span>
+<span tabIndex="1">baz</span>
+<span tabIndex="2">never really sure what goes after baz</span>
+```
+
+## Accessibility guidelines
+- [WCAG 2.4.3](https://www.w3.org/WAI/WCAG21/Understanding/focus-order)
+
+### Resources
+- [Chrome Audit Rules, AX_FOCUS_03](https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#ax_focus_03)