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-strict-equal.md

@@ -0,0 +1,24 @@
+# Suggest using `toStrictEqual()` (`prefer-strict-equal`)
+
+`toStrictEqual` not only checks that two objects contain the same data but also
+that they have the same structure. It is common to expect objects to not only
+have identical values but also to have identical keys. A stricter equality will
+catch cases where two objects do not have identical keys.
+
+## Rule details
+
+This rule triggers a warning if `toEqual()` is used to assert equality.
+
+### Default configuration
+
+The following pattern is considered warning:
+
+```js
+expect({ a: 'a', b: undefined }).toEqual({ a: 'a' }); // true
+```
+
+The following pattern is not warning:
+
+```js
+expect({ a: 'a', b: undefined }).toStrictEqual({ a: 'a' }); // false
+```