hyperlinkAt method
URL-launcher helper: returns the hyperlink URI at (row, col), or null.
Implementation
String? hyperlinkAt(int row, int col) {
if (_client == null) return null;
if (row >= _grid.rows || col >= _grid.columns) return null;
if (!isHyperlink(_grid.flagsAt(row, col))) return null;
return _client!.resolveHyperlink(_grid.hyperlinkIdAt(row, col));
}