feat(analytics): 增强 Google Analytics 隐私合规与追踪功能

- 修复 allow_google_signals 配置为 false，禁用跨设备追踪
- 升级 Cookie 同意组件，支持三级偏好控制（必要/分析/营销）
- 新增滚动深度追踪组件，追踪 25%/50%/75%/100% 里程碑
- 更新隐私政策，新增 Cookie 和网站分析工具章节
- 新增细化同意管理函数，支持 PIPL 合规

src/components/analytics/ScrollDepthTracker.tsx

@@ -0,0 +1,42 @@
+'use client';
+
+import { useEffect, useRef, useCallback } from 'react';
+import { usePathname } from 'next/navigation';
+import { trackScrollDepth } from '@/lib/analytics';
+
+const MILESTONES = [25, 50, 75, 100] as const;
+
+export function ScrollDepthTracker() {
+  const trackedRef = useRef<Set<number>>(new Set());
+  const pathname = usePathname();
+
+  const handleScroll = useCallback(() => {
+    const scrollTop = window.scrollY;
+    const docHeight = document.documentElement.scrollHeight - window.innerHeight;
+
+    if (docHeight <= 0) {
+      return;
+    }
+
+    const scrollPercent = Math.round((scrollTop / docHeight) * 100);
+
+    MILESTONES.forEach((milestone) => {
+      if (scrollPercent >= milestone && !trackedRef.current.has(milestone)) {
+        trackedRef.current.add(milestone);
+        trackScrollDepth(milestone);
+      }
+    });
+  }, []);
+
+  useEffect(() => {
+    trackedRef.current = new Set();
+
+    window.addEventListener('scroll', handleScroll, { passive: true });
+
+    return () => {
+      window.removeEventListener('scroll', handleScroll);
+    };
+  }, [pathname, handleScroll]);
+
+  return null;
+}