**Steps to reproduce:**
* Install the **Accounting** module.
* Unhide the **Start Date** and **End Date** fields on invoice lines.
* Create and post a customer invoice with deferred dates.
* Go to **Accounting → Reports → Deferred Revenue Report**.
* Add an annotation to a deferred revenue line by clicking the **annotate** from three dots next to the account.
* Export the report in **XLSX** format.

**Observed behavior:**
* The export fails with a server error: `UnboundLocalError: cannot access local variable 'annotations_x_offset' where it is not associated with a value`

**Cause:**
* The variable `annotations_x_offset` is assigned inside the `for header_level_index, header_level in enumerate(options['column_headers'])` loop, which writes the "Annotations" column header for each header level.
* The Deferred Revenue Report produces an empty `column_headers` list, so the loop body never executes and `annotations_x_offset` is never assigned.
* When the code later tries to write annotation data for each report line, it references the unassigned variable, causing Python to raise `UnboundLocalError`.

**Fix:**
* Introduce a boolean flag `annotations_header_written = False` before the header loop to explicitly track whether the "Annotations" column header has already been written.
* Inside the header loop, set `annotations_header_written = True` after writing the header.
* After writing all individual column headers (where `x_offset` already points to the first free column after all data columns), add a fallback: if `report_annotations` is set but `annotations_header_written` is still `False`, assign `annotations_x_offset` from the current `x_offset` and write the "Annotations" header.

opw-6354473

Forward-Port-Of: odoo/enterprise#122768