ui: drop the 'download complete' success toast

Successful downloads no longer pop a 'downloadOk' toast across all
the download sites (PopularRankList, RecommendedCard, FileDocBubble,
ImageLightbox). The browser's native download UI already confirms
the action, and the toast was redundant noise. Failure toasts and
the spinner state remain so users still see errors.

src/components/messageStream/overlays/ImageLightbox.tsx

@@ -94,7 +94,6 @@ function LightboxDownloadButton({
     setIsDownloading(true);
     try {
       await downloadAttachment(postId, attachment.id, attachment.filename);
-      showToast(t("downloadOk"));
     } catch {
       showToast(t("downloadFail"), "error");
     } finally {