remove import / export on mobile

frontend/src/App.tsx

@@ -108,18 +108,6 @@ function Navigation({ onImportExportClick, onAboutClick }: { onImportExportClick
               
               {/* Mobile Action Buttons */}
               <div className="border-t border-gray-200 pt-3 mt-3 space-y-1">
-                <button
-                  onClick={() => {
-                    onImportExportClick();
-                    setIsMobileMenuOpen(false);
-                  }}
-                  className="block w-full text-left px-3 py-2 text-base font-medium text-gray-600 hover:text-blue-600 hover:bg-gray-50"
-                >
-                  <svg className="w-4 h-4 inline mr-2" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M7 16a4 4 0 01-.88-7.903A5 5 0 1115.9 6L16 6a5 5 0 011 9.9M9 19l3 3m0 0l3-3m-3 3V10" />
-                  </svg>
-                  Import / Export
-                </button>
                 <button
                   onClick={() => {
                     onAboutClick();