Add comprehensive visitor engagement tracking and scroll-to-top on navigation

Tracks scroll depth (25/50/75/90%), time on page, UTM parameters, outbound
link clicks, search queries, audio pause/completion/listen time, and 404s.
Logged-in study users are now tied to their visitor record and surfaced in
the admin recent visits table. Scroll position resets on every route change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.js

@@ -22,6 +22,7 @@ import {
   loadQrCodesFromDisk,
   loadEpisodePlaysFromDisk,
   loadPodcastChecklistFromDisk,
+  loadAnalyticsEventsFromDisk,
   createBackupSnapshot,
   refreshContentCaches,
   queueHitStatsWrite,
@@ -115,6 +116,7 @@ Promise.all([
   loadQrCodesFromDisk(),
   loadEpisodePlaysFromDisk(),
   loadPodcastChecklistFromDisk(),
+  loadAnalyticsEventsFromDisk(),
   refreshContentCaches(),
 ])
   .catch(err => {