fix(pipelines): make the Delete Pipeline button text visible

The button combined `variant="destructive"` — which is the soft treatment,
`bg-destructive/10 text-destructive` — with a `bg-red-600` override. The
override replaced the tinted background but not the red text, leaving red
on red and no readable label.

Drop the variant and add `text-white`, which is how every other solid-red
button in the app is written, including the delete-confirm button 100
lines up in this same dialog.

Closes #420.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/components/pipelines/pipeline-settings.tsx

@@ -337,9 +337,8 @@ export function PipelineSettings({
 
             <DialogFooter className="border-border bg-popover/50">
               <Button
-                variant="destructive"
                 onClick={() => setShowDeleteConfirm(true)}
-                className="mr-auto bg-red-600 hover:bg-red-700"
+                className="mr-auto bg-red-600 text-white hover:bg-red-700"
               >
                 {t("deletePipeline")}
               </Button>