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-called-with.md

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+# Suggest using `toBeCalledWith()` or `toHaveBeenCalledWith()` (`prefer-called-with`)
+
+The `toBeCalled()` matcher is used to assert that a mock function has been
+called one or more times, without checking the arguments passed. The assertion
+is stronger when arguments are also validated using the `toBeCalledWith()`
+matcher. When some arguments are difficult to check, using generic match like
+`expect.anything()` at least enforces number and position of arguments.
+
+This rule warns if the form without argument checking is used, except for `.not`
+enforcing a function has never been called.
+
+## Rule details
+
+The following patterns are warnings:
+
+```js
+expect(someFunction).toBeCalled();
+
+expect(someFunction).toHaveBeenCalled();
+```
+
+The following patterns are not warnings:
+
+```js
+expect(noArgsFunction).toBeCalledWith();
+
+expect(roughArgsFunction).toBeCalledWith(expect.anything(), expect.any(Date));
+
+expect(anyArgsFunction).toBeCalledTimes(1);
+
+expect(uncalledFunction).not.toBeCalled();
+```