perf(banner): smoother deep-link from banner to /browse post

- MessageStream: drop the mount-time scroll lock and the 80ms-delayed
  custom rAF; engage the lock only while the smooth animation runs and
  use native scrollTo({behavior:'smooth'}) so the page never feels frozen
  during pagination and the easing is buttery.
- PublicLayout: fire the default /browse prefetch immediately on mount
  (banner / Home tile destination) so a fast tap hits a warm cache;
  popular / latest stay deferred to idle.
- FigmaBanner: prefetch the all-scope stream on mount and on pointerdown
  as safety nets, and ignore empty / '#' / javascript: link URLs so a
  contentless banner renders as a non-interactive image.
- usePostStream: dedupe in-flight prefetches by key so concurrent
  callers (layout + banner) collapse into a single network request.

src/components/FigmaBanner.tsx

@@ -12,6 +12,7 @@ import { assetUrl, getJSON, itemsOrEmpty, readJSONCache } from "../api";
 import { EASE_OUT } from "../motion";
 import { langQuery, useI18n, type Lang } from "../i18n";
 import { localizePath, stripLangPrefix } from "../languageRoutes";
+import { prefetchPostStream } from "./messageStream/hooks/usePostStream";
 
 const FIGMA_ASSET_BASE = "/assets/ark-library/figma";
 
@@ -87,12 +88,17 @@ function toSlides(items: BannerApiItem[]): BannerSlide[] {
     .slice(0, MAX_BANNERS)
     .map((item) => {
       const imageUrl = assetUrl(item.imageUrl);
+      const raw = (item.linkUrl ?? "").trim();
+      // Treat empty, placeholder, or javascript: URLs as "no link" so the
+      // slide renders as a non-interactive image rather than a dead anchor.
+      const linkUrl =
+        raw && raw !== "#" && !/^javascript:/i.test(raw) ? raw : undefined;
       return {
         id: String(item.id),
         mobile: imageUrl,
         desktop: imageUrl,
         alt: "",
-        linkUrl: item.linkUrl || undefined,
+        linkUrl,
       };
     });
 }
@@ -138,6 +144,32 @@ export function FigmaBanner() {
     };
   }, [lang]);
 
+  // Banner clicks land on /browse?post=<id>. Warm the all-scope stream cache
+  // the moment the banner is on screen so the destination renders instantly
+  // instead of showing a skeleton / loading state before scrolling to the post.
+  useEffect(() => {
+    prefetchPostStream({
+      scope: { kind: "all" },
+      type: "all",
+      q: "",
+      sort: "",
+      lang,
+    });
+  }, [lang]);
+
+  // Safety net: if the user taps quickly before mount-prefetch completes,
+  // pointerdown re-issues the prefetch. Same-key requests are deduped by the
+  // stream cache, so this is free when the warm-up already ran.
+  const prefetchAllStream = useCallback(() => {
+    prefetchPostStream({
+      scope: { kind: "all" },
+      type: "all",
+      q: "",
+      sort: "",
+      lang,
+    });
+  }, [lang]);
+
   const goTo = useCallback((index: number, behavior: ScrollBehavior) => {
     const scroller = scrollerRef.current;
     if (!scroller) return;
@@ -217,6 +249,7 @@ export function FigmaBanner() {
   }, [hasMultiple, autoplayPaused, publicMenuOpen, slides.length, goTo]);
 
   const handlePointerDown = (event: ReactPointerEvent<HTMLDivElement>) => {
+    prefetchAllStream();
     if (event.pointerType !== "mouse") return;
     const scroller = scrollerRef.current;
     if (!scroller) return;