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/valid-expect-in-promise.md

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+# Ensure promises that have expectations in their chain are valid (`valid-expect-in-promise`)
+
+Ensure promises that include expectations are returned or awaited.
+
+## Rule details
+
+This rule flags any promises within the body of a test that include expectations
+that have either not been returned or awaited.
+
+The following patterns is considered warning:
+
+```js
+it('promises a person', () => {
+  api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+});
+
+it('promises a counted person', () => {
+  const promise = api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+
+  promise.then(() => {
+    expect(analytics.gottenPeopleCount).toBe(1);
+  });
+});
+
+it('promises multiple people', () => {
+  const firstPromise = api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+  const secondPromise = api.getPersonByName('alice').then(person => {
+    expect(person).toHaveProperty('name', 'Alice');
+  });
+
+  return Promise.any([firstPromise, secondPromise]);
+});
+```
+
+The following pattern is not warning:
+
+```js
+it('promises a person', async () => {
+  await api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+});
+
+it('promises a counted person', () => {
+  let promise = api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+
+  promise = promise.then(() => {
+    expect(analytics.gottenPeopleCount).toBe(1);
+  });
+
+  return promise;
+});
+
+it('promises multiple people', () => {
+  const firstPromise = api.getPersonByName('bob').then(person => {
+    expect(person).toHaveProperty('name', 'Bob');
+  });
+  const secondPromise = api.getPersonByName('alice').then(person => {
+    expect(person).toHaveProperty('name', 'Alice');
+  });
+
+  return Promise.allSettled([firstPromise, secondPromise]);
+});
+```