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/motion/variants.ts

@@ -5,7 +5,7 @@ export const EASE_OUT = [0.22, 1, 0.36, 1] as const;
 
 /** Base transition for reveal-style animations. */
 export const baseTransition: Transition = {
-  duration: 0.4,
+  duration: 0.25,
   ease: EASE_OUT,
 };