Fix: Import missing 'cn' and 'X' in App.tsx. Resolves blank screen runtime error.

src/App.tsx

@@ -1,5 +1,7 @@
 import React, { useState, useMemo, useEffect } from 'react';
 import * as XLSX from 'xlsx';
+import { X } from 'lucide-react';
+import { cn } from './lib/utils';
 import { AppState, ExcelRow, resolveColumnIndices, COLUMNS } from './types';
 import { ColumnsProvider } from './contexts/ColumnsContext';
 import { Sidebar } from './components/Sidebar';