// Lee TODOS los libros (pestañas) del Google Sheet
function doGet() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheets = ss.getSheets();
  var allRows = [];
  var globalHeaders = null;

  sheets.forEach(function(sheet) {
    var data = sheet.getDataRange().getValues();
    if (data.length < 2) return;
    var headers = data[0].map(function(h) { return String(h).trim(); });
    if (!globalHeaders) globalHeaders = headers;
    data.slice(1).forEach(function(row) {
      var obj = { '__libro': sheet.getName() };
      headers.forEach(function(h, i) {
        obj[h] = (row[i] !== undefined && row[i] !== null) ? row[i] : '';
      });
      allRows.push(obj);
    });
  });

  if (!allRows.length) return ContentService.createTextOutput('').setMimeType(ContentService.MimeType.CSV);

  var cols = ['__libro'].concat(globalHeaders || []);
  function esc(v) {
    var s = String(v === null || v === undefined ? '' : v);
    return (s.indexOf(',') >= 0 || s.indexOf('"') >= 0 || s.indexOf('\n') >= 0)
      ? '"' + s.replace(/"/g, '""') + '"' : s;
  }
  var csv = [cols.map(esc).join(',')].concat(
    allRows.map(function(r) { return cols.map(function(c) { return esc(r[c]); }).join(','); })
  ).join('\n');

  return ContentService.createTextOutput(csv).setMimeType(ContentService.MimeType.CSV);
}