Renders a single variable widget for the toolbar.

Props:

• var - %QueryVariable{} or map with keys:

                  :name, :type (:text|:number|:date),
                  :widget (:input|:select|:date),
                  :label, :default, :static_options

• value - current value override (from @variable_values); falls back to var.default
• class - extra classes for outer wrapper
• resolved_options - pre-resolved Phoenix select options [{label, value}] for SQL-based dropdowns;

                  takes precedence over static_options when provided