perf: perf the control logic of `VbenModal` full screen and header (#6566)

* resolve the issue of header=false and full screen button display but not operable

packages/@core/ui-kit/popup-ui/src/modal/modal.vue

@@ -98,9 +98,7 @@ const {
   zIndex,
 } = usePriorityValues(props, state);
 
-const shouldFullscreen = computed(
-  () => (fullscreen.value && header.value) || isMobile.value,
-);
+const shouldFullscreen = computed(() => fullscreen.value || isMobile.value);
 
 const shouldDraggable = computed(
   () => draggable.value && !shouldFullscreen.value && header.value,