Apps Script Code — paste this into your script.google.com project

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const TOKEN = 'excelspine2026';

function doGet(e) {
  const p = e && e.parameter;
  if (!p || p.token !== TOKEN) return json({ error: 'unauthorized' });
  const ss = SpreadsheetApp.getActiveSpreadsheet();

  if (p.action === 'tasks') {
    const gtdSheet = ss.getSheetByName('GTD');
    if (gtdSheet) {
      const raw = gtdSheet.getRange('A1').getValue();
      if (raw) { try { return json({ tasks: JSON.parse(raw) }); } catch(e) {} }
    }
    const taskSheet = ss.getSheetByName('Tasks');
    if (!taskSheet) return json({ tasks: [] });
    const data = taskSheet.getDataRange().getValues();
    if (data.length < 2) return json({ tasks: [] });
    const headers = data[0];
    const tasks = data.slice(1).map(row => {
      const t = {};
      headers.forEach((h, i) => { t[h] = row[i] || ''; });
      ['due_date','date_added'].forEach(f => {
        if (t[f] instanceof Date) {
          const d = t[f];
          t[f] = d.getFullYear()+'-'+String(d.getMonth()+1).padStart(2,'0')+'-'+String(d.getDate()).padStart(2,'0');
        } else if (t[f] && String(t[f]).includes('GMT')) {
          const d = new Date(t[f]);
          if (!isNaN(d)) t[f] = d.getFullYear()+'-'+String(d.getMonth()+1).padStart(2,'0')+'-'+String(d.getDate()).padStart(2,'0');
        }
      });
      if (t.due_time && String(t.due_time).includes('1899')) t.due_time = '';
      t.done = t.done === 'Yes' || t.done === true;
      t.calendarSynced = !!t.cal_event_id;
      return t;
    });
    return json({ tasks: tasks });
  }

  if (p.action === 'gtd_versions') {
    const sheet = ss.getSheetByName('GTD_Versions');
    if (!sheet) return json({ versions: [] });
    const data = sheet.getDataRange().getValues();
    if (data.length < 2) return json({ versions: [] });
    const versions = data.slice(1).map(row => ({ id: row[0], timestamp: row[1], label: row[2], snapshot: row[3] }));
    return json({ versions: versions });
  }
  return json({ error: 'unknown action' });
}

function doPost(e) {
  const body = JSON.parse(e.postData.contents);
  if (body.token !== TOKEN) return json({ error: 'unauthorized' });
  const ss = SpreadsheetApp.getActiveSpreadsheet();

  if (body.action === 'sync_tasks') {
    const sheet = ss.getSheetByName('GTD') || ss.insertSheet('GTD');
    sheet.getRange('A1').setValue(JSON.stringify(body.tasks));
    sheet.getRange('B1').setValue(new Date().toISOString());
    writeTasksTab(ss, body.tasks);
    return json({ status: 'ok' });
  }

  if (body.action === 'save_gtd_version') {
    const sheet = ss.getSheetByName('GTD_Versions') || ss.insertSheet('GTD_Versions');
    const v = body.version;
    sheet.appendRow([v.id, v.timestamp, v.label, v.snapshot]);
    return json({ status: 'ok' });
  }

  if (body.action === 'create_gtd_cal_event') {
    const task = body.task;
    if (!task || !task.due_date) return json({ error: 'no due_date' });
    const cal = CalendarApp.getDefaultCalendar();
    const start = new Date(task.due_date);
    const end = new Date(start.getTime() + 60 * 60 * 1000);
    const desc = (task.note || '') + '\n\n[GTD][id:' + task.id + ']';
    const event = cal.createEvent('[GTD] ' + task.title, start, end, { description: desc });
    return json({ status: 'ok', calEventId: event.getId() });
  }
  return json({ error: 'unknown action' });
}

function writeTasksTab(ss, tasks) {
  const tab = ss.getSheetByName('Tasks') || ss.insertSheet('Tasks');
  tab.clearContents();
  const headers = ['id','title','due_date','due_time','note','gtd_decision','date_added','cal_event_id','done'];
  const rows = [headers];
  (tasks||[]).forEach(t => {
    rows.push([t.id||'',t.title||'',t.due_date||'',t.due_time||'',t.note||'',t.gtd_decision||'',t.date_added||'',t.cal_event_id||'',t.done?'Yes':'No']);
  });
  tab.getRange(1,1,rows.length,headers.length).setValues(rows);
  tab.getRange(1,1,1,headers.length).setFontWeight('bold');
  tab.setFrozenRows(1);
}

function json(obj) {
  return ContentService.createTextOutput(JSON.stringify(obj)).setMimeType(ContentService.MimeType.JSON);
}

Deploy steps:
1. Open script.google.com → open the GTD project
2. Replace all code with the above → Save
3. Deploy → Manage Deployments → Edit → New Version → Deploy
4. Execute as: Me · Access: Anyone
5. Paste the Web App URL above → hit Test