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-react/lib/rules/state-in-constructor.js

@@ -0,0 +1,68 @@
+/**
+ * @fileoverview Enforce the state initialization style to be either in a constructor or with a class property
+ * @author Kanitkorn Sujautra
+ */
+
+'use strict';
+
+const astUtil = require('../util/ast');
+const componentUtil = require('../util/componentUtil');
+const docsUrl = require('../util/docsUrl');
+const report = require('../util/report');
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+const messages = {
+  stateInitConstructor: 'State initialization should be in a constructor',
+  stateInitClassProp: 'State initialization should be in a class property',
+};
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Enforce class component state initialization style',
+      category: 'Stylistic Issues',
+      recommended: false,
+      url: docsUrl('state-in-constructor'),
+    },
+
+    messages,
+
+    schema: [{
+      enum: ['always', 'never'],
+    }],
+  },
+
+  create(context) {
+    const option = context.options[0] || 'always';
+    return {
+      'ClassProperty, PropertyDefinition'(node) {
+        if (
+          option === 'always'
+          && !node.static
+          && node.key.name === 'state'
+          && componentUtil.getParentES6Component(context, node)
+        ) {
+          report(context, messages.stateInitConstructor, 'stateInitConstructor', {
+            node,
+          });
+        }
+      },
+      AssignmentExpression(node) {
+        if (
+          option === 'never'
+          && componentUtil.isStateMemberExpression(node.left)
+          && astUtil.inConstructor(context, node)
+          && componentUtil.getParentES6Component(context, node)
+        ) {
+          report(context, messages.stateInitClassProp, 'stateInitClassProp', {
+            node,
+          });
+        }
+      },
+    };
+  },
+};