Bold-format a Google Sheet's header row and freeze it so it stays visible while scrolling.

Documented steps

1. POST https://sheets.googleapis.com/v4/spreadsheets/{spreadsheetId}:batchUpdate using OAuth scope https://www.googleapis.com/auth/spreadsheets (or https://www.googleapis.com/auth/drive.file if limited to app-created files).
2. Add requests[0].repeatCell with range = GridRange{sheetId, startRowIndex:0, endRowIndex:1} — GridRange indexes are zero-based and half-open (start index inclusive, end index exclusive), so this covers only row 1.
3. Set requests[0].repeatCell.cell.userEnteredFormat.textFormat.bold = true and requests[0].repeatCell.fields = "userEnteredFormat.textFormat.bold" (the fields mask is required; only listed fields are written).
4. Add requests[1].updateSheetProperties.properties = {sheetId, gridProperties:{frozenRowCount:1}} and requests[1].updateSheetProperties.fields = "gridProperties.frozenRowCount".
5. Send both requests together in one requests[] array on a single batchUpdate call so they apply atomically.
6. Docs: https://developers.google.com/workspace/sheets/api/reference/rest/v4/spreadsheets/request | https://developers.google.com/workspace/sheets/api/samples/formatting | https://developers.google.com/workspace/sheets/api/limits