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/jsx-props-no-spread-multi.js

@@ -0,0 +1,54 @@
+/**
+ * @fileoverview Prevent JSX prop spreading the same expression multiple times
+ * @author Simon Schick
+ */
+
+'use strict';
+
+const docsUrl = require('../util/docsUrl');
+const report = require('../util/report');
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+const messages = {
+  noMultiSpreading: 'Spreading the same expression multiple times is forbidden',
+};
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Disallow JSX prop spreading the same identifier multiple times',
+      category: 'Best Practices',
+      recommended: false,
+      url: docsUrl('jsx-props-no-spread-multi'),
+    },
+    messages,
+  },
+
+  create(context) {
+    return {
+      JSXOpeningElement(node) {
+        const spreads = node.attributes.filter(
+          (attr) => attr.type === 'JSXSpreadAttribute'
+          && attr.argument.type === 'Identifier'
+        );
+        if (spreads.length < 2) {
+          return;
+        }
+        // We detect duplicate expressions by their identifier
+        const identifierNames = new Set();
+        spreads.forEach((spread) => {
+          if (identifierNames.has(spread.argument.name)) {
+            report(context, messages.noMultiSpreading, 'noMultiSpreading', {
+              node: spread,
+            });
+          }
+          identifierNames.add(spread.argument.name);
+        });
+      },
+    };
+  },
+};