fix: scope back-to-top to browse, speed up reveal, reset scroll on sort change

- BackToTop now only mounts on the /browse feed (covers all / latest /
  popular / search) instead of every route.
- Reveal animation duration cut 0.4s -> 0.25s so scrolled-in content
  appears faster.
- ScrollToTop also watches `search`, so switching between sort views on
  the same /browse path (e.g. 全部资料 <-> 热门资料) returns to the top.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/ScrollToTop.tsx

@@ -11,12 +11,12 @@ import { useLocation } from "react-router-dom";
  * anchor / deep-link targets keep their own scroll handling.
  */
 export function ScrollToTop() {
-  const { pathname, hash } = useLocation();
+  const { pathname, search, hash } = useLocation();
 
   useEffect(() => {
     if (hash) return;
     window.scrollTo({ top: 0, left: 0 });
-  }, [pathname, hash]);
+  }, [pathname, search, hash]);
 
   return null;
 }