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-have-length.md

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+# Suggest using `toHaveLength()` (`prefer-to-have-length`)
+
+In order to have a better failure message, `toHaveLength()` should be used upon
+asserting expectations on objects length property.
+
+## Rule details
+
+This rule triggers a warning if `toBe()`, `toEqual()` or `toStrictEqual()` is
+used to assert objects length property.
+
+```js
+expect(files.length).toBe(1);
+```
+
+This rule is enabled by default.
+
+### Default configuration
+
+The following patterns are considered warnings:
+
+```js
+expect(files.length).toBe(1);
+
+expect(files.length).toEqual(1);
+
+expect(files.length).toStrictEqual(1);
+```
+
+The following pattern is not warning:
+
+```js
+expect(files).toHaveLength(1);
+```