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-identical-title.md

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+# Disallow identical titles (`no-identical-title`)
+
+Having identical titles for two different tests or test suites may create
+confusion. For example, when a test with the same title as another test in the
+same test suite fails, it is harder to know which one failed and thus harder to
+fix.
+
+## Rule Details
+
+This rule looks at the title of every test and test suites. It will report when
+two test suites or two test cases at the same level of a test suite have the
+same title.
+
+The following patterns are considered warnings:
+
+```js
+describe('foo', () => {
+  it('should do bar', () => {});
+  it('should do bar', () => {}); // Has the same title as the previous test
+
+  describe('baz', () => {
+    // ...
+  });
+
+  describe('baz', () => {
+    // Has the same title as a previous test suite
+    // ...
+  });
+});
+```
+
+These patterns would not be considered warnings:
+
+```js
+describe('foo', () => {
+  it('should do foo', () => {});
+  it('should do bar', () => {});
+
+  // Has the same name as a parent test suite, which is fine
+  describe('foo', () => {
+    // Has the same name as a test in a parent test suite, which is fine
+    it('should do foo', () => {});
+    it('should work', () => {});
+  });
+
+  describe('baz', () => {
+    // Has the same title as a previous test suite
+    // Has the same name as a test in a sibling test suite, which is fine
+    it('should work', () => {});
+  });
+});
+```