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/layouts/PublicLayout.tsx

@@ -669,7 +669,7 @@ export function PublicLayout() {
         </div>
       </nav>
 
-      <BackToTop />
+      {pathname === "/browse" ? <BackToTop /> : null}
     </div>
   );
 }