Use single-choice controls for single-choice backlog filters

What would you like to change?

In the backlog filter menu, Priority, Assignee, and Due date use checkbox-style controls. They appear to allow several values, but selecting another option in the same section replaces the previous choice. This makes the filter behavior harder to understand.

Could single-choice sections use radio buttons or another control that is visibly exclusive? Checkboxes should remain for filters that support multiple selections. Each control should reflect the filter's actual behavior, including how a selection is cleared.

Additional context

The backlog stores one priority, one assignee, and one due-date preset, but renders those options with DropdownMenuCheckboxItem. The board filter state supports arrays for several fields, so this does not apply to every filter. It only changes the controls for sections that already behave as single choice. Any future multi-select filters can continue to use checkboxes.

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Originally requested by @thriz0 on 2026-09-28. Original GitHub request #1848.