honor explicit close-on-select=false on single-select comboboxes

Previously shouldCloseOnSelect used `||`, so an explicit
:close-on-select="false" on a single-select combobox was treated as
unset and fell back to "close on select". Switch to `??` so an explicit
false (or true) is always respected, and only an unset prop defers to
the multi-aware default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jasonvarga

resources/js/components/ui/Combobox/Combobox.vue

@@ -202,7 +202,7 @@ const limitIndicatorColor = computed(() => {
 });
 
 const canClearSelection = computed(() => props.clearable && props.modelValue);
-const shouldCloseOnSelect = computed(() => props.closeOnSelect || !props.multiple);
+const shouldCloseOnSelect = computed(() => props.closeOnSelect ?? !props.multiple);
 const shouldShowOptionsChevron = computed(() => props.options.length > 0 || props.ignoreFilter);
 const shouldShowLimitIndicator = computed(() => props.multiple && props.maxSelections && props.maxSelections !== Infinity);