fix: use text-foreground for the Dialog close icon

The close icon used the Button "secondary" variant's
text-secondary-foreground, a near-white token fixed across both
themes. That reads fine against the dark Hades popover but is almost
invisible against the light Apollo popover since bg-transparent
removes the variant's own background. text-foreground already adapts
per-theme and is what a plain icon-only dismiss button should use.
Pre-existing bug, unrelated to the reka-ui removal (same classes
existed before) -- confirmed via computed-style + screenshot checks in
both themes in headless Chromium.

packages/library/src/components/ui/dialog/DialogContent.vue

@@ -52,7 +52,7 @@ watch(context.open, (value) => { isLocked.value = value }, { immediate: true })
         <slot />
 
         <DialogClose v-if="showCloseButton" as-child>
-          <Button variant="secondary" class="absolute top-2 right-2 size-7 bg-transparent hover:bg-black/10" size="icon">
+          <Button variant="secondary" class="absolute top-2 right-2 size-7 bg-transparent text-foreground hover:bg-black/10" size="icon">
             <XIcon class="size-3.5" />
             <span class="sr-only">Close</span>
           </Button>