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/app/layout.tsx

@@ -8,6 +8,7 @@ import { GoogleAnalytics } from "@/components/analytics/GoogleAnalytics";
 import { CookieConsent } from "@/components/analytics/CookieConsent";
 import { PerformanceTracker } from "@/components/analytics/PerformanceTracker";
 import { OutboundLinkTracker } from "@/components/analytics/OutboundLinkTracker";
+import { ScrollDepthTracker } from "@/components/analytics/ScrollDepthTracker";
 import { OrganizationSchema, WebsiteSchema } from "@/components/seo/structured-data";
 import { MobileTabBar } from "@/components/layout/mobile-tab-bar";
 import { ErrorBoundary } from "@/components/ui/error-boundary";
@@ -141,6 +142,7 @@ export default function RootLayout({
         <GoogleAnalytics />
         <PerformanceTracker />
         <OutboundLinkTracker />
+        <ScrollDepthTracker />
         <ThemeProvider>
           <ErrorBoundary>
             {children}