Exporting report with tags to Excel fails

[jkl1337]

Attempting to export a report containing a frozenset column (so tags at least) to Excel causes a 500 error.

Exception on /beancount/download-query/query_result.xlsx [GET]
Traceback (most recent call last):
  File "/home/hambone/.local/share/uv/tools/fava/lib/python3.13/site-packages/openpyxl/worksheet/_write_only.py", line 133, in _values_to_row
    cell.value = value
    ^^^^^^^^^^
  File "/home/hambone/.local/share/uv/tools/fava/lib/python3.13/site-packages/openpyxl/cell/cell.py", line 218, in value
    self._bind_value(value)
    ~~~~~~~~~~~~~~~~^^^^^^^
  File "/home/hambone/.local/share/uv/tools/fava/lib/python3.13/site-packages/openpyxl/cell/cell.py", line 187, in _bind_value
    raise ValueError("Cannot convert {0!r} to Excel".format(value))
ValueError: Cannot convert frozenset() to Excel

Looks like this might be straightforwardish to fix. I'll look into later.

Additionally if a tags column that is empty gets exported as CSV it comes out as frozenset().

[yagebu]

A PR with a fix would be very welcome :)