Initial frontend implementation

frontend/src/App.js

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+import React, { useState } from 'react';
+import Editor from './components/Editor';
+import FileTree from './components/FileTree';
+import './App.scss';
+
+function App() {
+  const [content, setContent] = useState('# Welcome to NovaMD\n\nStart editing here!');
+  const [files, setFiles] = useState(['README.md', 'chapter1.md', 'chapter2.md']);
+
+  return (
+    <div className="App">
+      <div className="sidebar">
+        <FileTree files={files} />
+      </div>
+      <div className="main-content">
+        <h1>NovaMD</h1>
+        <Editor content={content} onChange={setContent} />
+      </div>
+    </div>
+  );
+}
+
+export default App;