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

app/[locale]/not-found.tsx

@@ -1,9 +1,20 @@
 import { useTranslations } from 'next-intl';
 import { Container, Button, Heading } from '@/components/ui';
 import Scribble from '@/components/Scribble';
+import { useEffect } from 'react';
+import { useAnalytics } from '@/components/analytics/useAnalytics';
+import { AnalyticsEvents } from '@/components/analytics/analytics-events';
 
 export default function NotFound() {
   const t = useTranslations('Error.notFound');
+  const { trackEvent } = useAnalytics();
+
+  useEffect(() => {
+    trackEvent(AnalyticsEvents.ERROR, {
+      type: '404_not_found',
+      path: typeof window !== 'undefined' ? window.location.pathname : 'unknown',
+    });
+  }, [trackEvent]);
 
   return (
     <Container className="relative py-24 flex flex-col items-center justify-center text-center min-h-[70vh] overflow-hidden">
@@ -16,19 +27,17 @@ export default function NotFound() {
         <Heading level={1} className="text-6xl md:text-8xl font-bold mb-2">
           404
         </Heading>
-        <Scribble 
-          variant="circle" 
-          className="w-[150%] h-[150%] -top-[25%] -left-[25%] text-accent/40" 
+        <Scribble
+          variant="circle"
+          className="w-[150%] h-[150%] -top-[25%] -left-[25%] text-accent/40"
         />
       </div>
 
       <Heading level={2} className="text-2xl md:text-3xl font-bold mb-4 text-primary">
         {t('title')}
       </Heading>
-      
-      <p className="text-white/60 mb-10 max-w-md text-lg">
-        {t('description')}
-      </p>
+
+      <p className="text-white/60 mb-10 max-w-md text-lg">{t('description')}</p>
 
       <div className="flex flex-col sm:flex-row gap-4">
         <Button href="/" variant="accent" size="lg">