feat(analytics): add blog engagement, ToC tracking, and 404 monitoring

- Added BlogEngagementTracker for reading time and completion tracking
- Added ToC click tracking in blog posts
- Added global 404 error monitoring in not-found.tsx
- Completed 'Total Transparency' suite

components/analytics/BlogEngagementTracker.tsx

@@ -0,0 +1,53 @@
+'use client';
+
+import { useEffect } from 'react';
+import { useAnalytics } from './useAnalytics';
+import { AnalyticsEvents } from './analytics-events';
+
+interface BlogEngagementTrackerProps {
+  title: string;
+  slug: string;
+  category?: string;
+  readingTime: number;
+}
+
+/**
+ * BlogEngagementTracker
+ * Tracks reading time and article completion.
+ */
+export default function BlogEngagementTracker({
+  title,
+  slug,
+  category,
+  readingTime,
+}: BlogEngagementTrackerProps) {
+  const { trackEvent } = useAnalytics();
+
+  useEffect(() => {
+    // Article start
+    trackEvent(AnalyticsEvents.BLOG_POST_VIEW, {
+      title,
+      slug,
+      category,
+      estimated_reading_time: readingTime,
+      location: 'blog_post_pdp',
+    });
+
+    const startTime = Date.now();
+
+    return () => {
+      const dwellTime = Math.round((Date.now() - startTime) / 1000);
+
+      // We only consider it a "read" if they stay a reasonable amount of time
+      // or if they scroll (covered by ScrollDepthTracker)
+      trackEvent('blog_dwell_time', {
+        title,
+        slug,
+        seconds: dwellTime,
+        reading_time_completion: Math.min(100, Math.round((dwellTime / (readingTime * 60)) * 100)),
+      });
+    };
+  }, [title, slug, category, readingTime, trackEvent]);
+
+  return null;
+}