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/prefer-to-contain.md

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+# Suggest using `toContain()` (`prefer-to-contain`)
+
+In order to have a better failure message, `toContain()` should be used upon
+asserting expectations on an array containing an object.
+
+## Rule details
+
+This rule triggers a warning if `toBe()`, `toEqual()` or `toStrictEqual()` is
+used to assert object inclusion in an array
+
+```js
+expect(a.includes(b)).toBe(true);
+```
+
+```js
+expect(a.includes(b)).not.toBe(true);
+```
+
+```js
+expect(a.includes(b)).toBe(false);
+```
+
+### Default configuration
+
+The following patterns are considered warnings:
+
+```js
+expect(a.includes(b)).toBe(true);
+
+expect(a.includes(b)).not.toBe(true);
+
+expect(a.includes(b)).toBe(false);
+
+expect(a.includes(b)).toEqual(true);
+
+expect(a.includes(b)).toStrictEqual(true);
+```
+
+The following patterns are not considered warnings:
+
+```js
+expect(a).toContain(b);
+
+expect(a).not.toContain(b);
+```