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>

src/components/EpisodeAudioPlayer.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useRef, useState } from 'react'
+import { sendEvent } from '../analytics'
 
 interface EpisodeAudioPlayerProps {
   src: string
@@ -36,13 +37,30 @@ export function EpisodeAudioPlayer({ src, title, size = 'full', spotifyUrl }: Ep
   const [currentTime, setCurrentTime] = useState(0)
   const [duration, setDuration] = useState(0)
   const playTrackedRef = useRef(false)
+  const listenStartRef = useRef<number | null>(null)
+  const totalListenSecondsRef = useRef(0)
+
+  function flushListenTime() {
+    if (listenStartRef.current !== null && title) {
+      const seconds = Math.round((Date.now() - listenStartRef.current) / 1000)
+      if (seconds > 1) {
+        totalListenSecondsRef.current += seconds
+        sendEvent('audio_listen_time', { title, seconds })
+      }
+      listenStartRef.current = null
+    }
+  }
 
   useEffect(() => {
     const audio = audioRef.current
     if (!audio) return
     const onTimeUpdate = () => setCurrentTime(audio.currentTime)
     const onDurationChange = () => setDuration(audio.duration)
-    const onEnded = () => setPlaying(false)
+    const onEnded = () => {
+      setPlaying(false)
+      flushListenTime()
+      if (title) sendEvent('audio_completion', { title })
+    }
     audio.addEventListener('timeupdate', onTimeUpdate)
     audio.addEventListener('durationchange', onDurationChange)
     audio.addEventListener('loadedmetadata', onDurationChange)
@@ -52,7 +70,9 @@ export function EpisodeAudioPlayer({ src, title, size = 'full', spotifyUrl }: Ep
       audio.removeEventListener('durationchange', onDurationChange)
       audio.removeEventListener('loadedmetadata', onDurationChange)
       audio.removeEventListener('ended', onEnded)
+      flushListenTime()
     }
+  // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [src])
 
   function togglePlay() {
@@ -61,9 +81,12 @@ export function EpisodeAudioPlayer({ src, title, size = 'full', spotifyUrl }: Ep
     if (playing) {
       audio.pause()
       setPlaying(false)
+      flushListenTime()
+      if (title) sendEvent('audio_pause', { title })
     } else {
       audio.play().then(() => {
         setPlaying(true)
+        listenStartRef.current = Date.now()
         if (!playTrackedRef.current && title) {
           playTrackedRef.current = true
           fetch('/api/analytics/play', {