fix: fill sparse holes in extendedHeaders to prevent undefined.includes() crash

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -154,6 +154,10 @@ export default function App() {
               extendedHeaders[idx] = name;
             }
           });
+          // Fill any sparse holes with '' to avoid undefined.includes() errors
+          for (let i = 0; i < extendedHeaders.length; i++) {
+            if (extendedHeaders[i] === undefined) extendedHeaders[i] = '';
+          }
 
           const resolvedCols = resolveColumnIndices(extendedHeaders);
           const editableColumns = new Set([