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/no-duplicate-hooks.md

@@ -0,0 +1,75 @@
+# Disallow duplicate setup and teardown hooks (`no-duplicate-hooks`)
+
+A `describe` block should not contain duplicate hooks.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule
+
+```js
+/* eslint jest/no-duplicate-hooks: "error" */
+
+describe('foo', () => {
+  beforeEach(() => {
+    // some setup
+  });
+  beforeEach(() => {
+    // some setup
+  });
+  test('foo_test', () => {
+    // some test
+  });
+});
+
+// Nested describe scenario
+describe('foo', () => {
+  beforeEach(() => {
+    // some setup
+  });
+  test('foo_test', () => {
+    // some test
+  });
+  describe('bar', () => {
+    test('bar_test', () => {
+      afterAll(() => {
+        // some teardown
+      });
+      afterAll(() => {
+        // some teardown
+      });
+    });
+  });
+});
+```
+
+Examples of **correct** code for this rule
+
+```js
+/* eslint jest/no-duplicate-hooks: "error" */
+
+describe('foo', () => {
+  beforeEach(() => {
+    // some setup
+  });
+  test('foo_test', () => {
+    // some test
+  });
+});
+
+// Nested describe scenario
+describe('foo', () => {
+  beforeEach(() => {
+    // some setup
+  });
+  test('foo_test', () => {
+    // some test
+  });
+  describe('bar', () => {
+    test('bar_test', () => {
+      beforeEach(() => {
+        // some setup
+      });
+    });
+  });
+});
+```