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-jest/docs/rules/no-mocks-import.md

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+# Disallow manually importing from `__mocks__` (`no-mocks-import`)
+
+When using `jest.mock`, your tests (just like the code being tested) should
+import from `./x`, not `./__mocks__/x`. Not following this rule can lead to
+confusion, because you will have multiple instances of the mocked module:
+
+```js
+jest.mock('./x');
+const x1 = require('./x');
+const x2 = require('./__mocks__/x');
+
+test('x', () => {
+  expect(x1).toBe(x2); // fails! They are both instances of `./__mocks__/x`, but not referentially equal
+});
+```
+
+### Rule details
+
+This rule reports imports from a path containing a `__mocks__` component.
+
+Example violations:
+
+```js
+import thing from './__mocks__/index';
+require('./__mocks__/index');
+require('__mocks__');
+```