fix: 回到顶部按钮用 IntersectionObserver 检测+scrollIntoView

- 顶部放哨兵元素，离开视口时显示回到顶部按钮
- 点击用 scrollIntoView 替代 window.scrollTo，兼容各种布局

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/modules/mileage/MonitoringView.tsx

@@ -196,18 +196,20 @@ export default function MonitoringView() {
     return () => observer.disconnect();
   }, []);
 
-  // 回到顶部按钮
+  // 回到顶部按钮：用 IntersectionObserver 检测顶部哨兵是否离开视口
+  const topSentinelRef = useRef<HTMLDivElement>(null);
   useEffect(() => {
-    const handleScroll = () => {
-      const scrollY = window.scrollY || document.documentElement.scrollTop;
-      setShowBackToTop(scrollY > 400);
-    };
-    window.addEventListener('scroll', handleScroll, { passive: true });
-    return () => window.removeEventListener('scroll', handleScroll);
+    const el = topSentinelRef.current;
+    if (!el) return;
+    const observer = new IntersectionObserver(
+      ([entry]) => setShowBackToTop(!entry.isIntersecting),
+    );
+    observer.observe(el);
+    return () => observer.disconnect();
   }, []);
 
   const scrollToTop = () => {
-    window.scrollTo({ top: 0, behavior: 'smooth' });
+    topSentinelRef.current?.scrollIntoView({ behavior: 'smooth' });
   };
 
   const filteredVehicles = vehicles;
@@ -228,6 +230,9 @@ export default function MonitoringView() {
 
   return (
     <>
+      {/* 顶部哨兵：离开视口时显示回到顶部按钮 */}
+      <div ref={topSentinelRef} className="h-0" />
+
       {/* Fullscreen Landscape View Overlay */}
       <AnimatePresence>
         {isFullscreen && (