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/state.js

@@ -63,6 +63,27 @@ export const state = {
   episodePlays: {},
   episodePlaysWritePromise: Promise.resolve(),
 
+  // analyticsEvents: aggregated engagement data from frontend events
+  // {
+  //   scrollDepth: { [path]: { 25: n, 50: n, 75: n, 90: n } }
+  //   timeOnPage: { [path]: { totalSeconds: n, count: n } }
+  //   outboundClicks: { [url]: n }
+  //   utmSources: { [utm_source]: n }
+  //   searchQueries: { [query]: n }
+  //   notFound: { [path]: n }
+  //   audioEvents: { [title]: { pauses: n, completions: n, totalListenSeconds: n } }
+  // }
+  analyticsEvents: {
+    scrollDepth: {},
+    timeOnPage: {},
+    outboundClicks: {},
+    utmSources: {},
+    searchQueries: {},
+    notFound: {},
+    audioEvents: {},
+  },
+  analyticsEventsWritePromise: Promise.resolve(),
+
   // qrCodes: Array<{ id, slug, label, destination, createdAt }>
   // qrScans: Array<{ id, qrId, slug, scannedAt, ip, userAgent }>
   qrCodes: [],